Gain and signal level adjustments of cascaded optical amplifiers

ABSTRACT

An optical amplification device which includes first and second optical amplifiers, and a controller. The first optical amplifier receives a light and amplifies the received light. The second optical amplifier receives the light amplified by the first optical amplifier, and amplifies the received light. When a level of the light received by the first optical amplifier changes by Δ, the controller controls a level of the light received by the second optical amplifier to change by approximately −Δ. In various embodiments, the controller causes the sum of the gains of the first and second optical amplifiers to be constant. In other embodiments, the optical amplification device includes first and second optical amplifier and a gain adjustor. The gain adjustor detects a deviation in gain of the first optical amplifier from a target gain, and adjusts the gain of the second optical amplifier to compensate for the detected deviation.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is based on, and claims priority to, Japaneseapplication number 10-70251, filed on Mar. 19, 1998, in Japan, and whichis incorporated herein by reference.

[0002] This application is also based on, and claims priority to,Japanese application number 10-258114, filed on Sep. 11, 1998, in Japan,and which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to cascaded optical amplifiers and,more particularly, to gain and signal level adjustments of cascadedoptical amplifiers.

[0005] 2. Description of the Related Art

[0006] Optical communication systems using optical fiber transmissionlines are being used to transmit relatively large amounts ofinformation. For this purpose, low-loss (e.g., 0.2 dB/km) optical fibershave been manufactured and used as optical fiber transmission lines. Inaddition, optical amplifiers are used to compensate for losses in theoptical fiber transmission line to thereby allow long-haul transmission.

[0007] A conventional optical amplifier includes an optical amplifyingmedium which is pumped with pump light to provide a gain band. Theoptical amplifying medium and pump light are chosen so that they providea gain band which includes a wavelength of signal light. As a result,the signal light will be amplified as the signal light travels throughthe optical amplifying medium. For example, an erbium doped fiberamplifier (EDFA) includes an erbium doped fiber (EDF) as the opticalamplifying medium. A pumping light source supplies pump light having apredetermined wavelength to the EDF. By setting the wavelength of thepump light within a 0.98 μm band or a 1.48 μm band, a gain bandincluding a wavelength band of 1.55 μm can be obtained. Therefore,signal light in the 1.55 μm wavelength band will be amplified.

[0008] Another type of conventional optical amplifier has asemiconductor chip as the optical amplifying medium. In this case,pumping is performed by injecting an electric current into thesemiconductor chip.

[0009] Further, wavelength division multiplexing (WDM) is a knowntechnique for increasing transmission capacity through a single opticalfiber. In a system adopting WDM, a plurality of optical carriers havingdifferent wavelengths are individually modulated with data. Thus, eachmodulated carrier represents a channel of the WDM system transmitting anoptical signal. The optical signals (that is, the modulated carriers)are then wavelength division multiplexed by an optical multiplexer toobtain WDM signal light. The WDM signal light is then transmittedthrough an optical fiber transmission line. The WDM signal light isreceived through the transmission line, and then demultipexed intoindividual optical signals by an optical demultiplexer. Data can then bedetected from these individual optical signals. Therefore, by applyingWDM, the transmission capacity of a single optical fiber can beincreased in accordance with the number of WDM channels multiplexedtogether and transmitted through the optical fiber.

[0010] When an optical amplifier is inserted along the transmission linein an optical communication system adopting WDM, a transmission distanceis limited by the wavelength characteristic of gain which is representedby a gain tilt or gain deviation of the optical amplifier. For example,in an EDFA, it is known that a gain tilt is produced at wavelengths inthe vicinity of 1.55 μm, and this gain tilt varies with total inputpower of signal light and pump light power to the EDFA.

[0011] There is an optical amplification device for opticalamplification which can maintain the wavelength characteristic of gainconstant and obtain a wide input dynamic range. The opticalamplification device includes first and second optical amplifiers and avariable optical attenuator optically connected between the first andsecond optical amplifiers. Automatic gain control (AGC) is applied toeach of the first and second optical amplifiers, thereby maintainingconstant the wavelength characteristic of gain of each of the first andsecond optical amplifiers. Further, automatic output level control (ALC)is performed for the second optical amplifier by using the variableoptical attenuator, thereby obtaining a wide input dynamic range. Thatis, the output level of the second optical amplifier is maintainedconstant irrespective of the input level of the first optical amplifier,so that the input dynamic range of this device is widened.

[0012] In such an optical amplification device, AGC is performed so thatthe gain of the first optical amplifier is maintained constantirrespective of the input level of the first optical amplifier.Accordingly, there arises a problem such that when the power of signallight to be supplied to the first optical amplifier is increased, thepower of pump light must be increased by the corresponding amount toincrease the output power of the first optical amplifier to provide therequired gain. That is, a high-power pumping light source is requiredfor the first optical amplifier to ensure a required input dynamicrange.

SUMMARY OF THE INVENTION

[0013] Accordingly, it is an object of the present invention to providean optical amplification device and method which does not require ahigh-power pumping light source.

[0014] It is an additional object of the present invention to provide anoptical amplification device and method which can maintain thewavelength characteristic of gain constant and can obtain a wide inputdynamic range, while requiring a relatively low power pump light.

[0015] Additional objects and advantages of the invention will be setforth in part in the description which follows, and, in part, will beobvious from the description, or may be learned by practice of theinvention.

[0016] The foregoing objects of the present invention are achieved byproviding an apparatus comprising first and second optical amplifiers,and a controller. The first optical amplifier receives a light andamplifies the received light. The second optical amplifier receives thelight amplified by the first optical amplifier, and amplifies thereceived light. The controller, when a level of the light received bythe first optical amplifier changes by Δ, controls a level of the lightreceived by the second optical amplifier to change by approximately −Δ.

[0017] Objects of the present invention are further achieved byproviding an apparatus which includes first and second opticalamplifiers, a variable attenuator and a controller. The first opticalamplifier receives a light and amplifies the received light. The secondoptical amplifier receives the light amplified by the first opticalamplifier, and amplifies the received light. The variable attenuator isoptically connected between the first and second optical amplifiers. Thecontroller controls attenuation of the variable attenuator so that, whena level of the light received by the first optical amplifier changes byΔ, a level of the light received by the second optical amplifier changesby approximately −Δ.

[0018] Objects of the present invention are further achieved byproviding an optical communication system. In the optical communicationsystem, optical transmitters transmit optical signals at differentwavelengths. A multiplexer multiplexes the optical signals into awavelength division multiplexed (WDM) signal light. The WDM signal lightis transmitted through an optical fiber transmission line. An opticalamplification device amplifies the WDM signal light as the WDM signallight is transmitted through the transmission line. The opticalamplification device includes first and second optical amplifiers and acontroller. The first optical amplifier receives the WDM signal lightand amplifies the received WDM signal light. The second opticalamplifier receives the WDM signal light amplified by the first opticalamplifier, and amplifies the received WDM signal light. The controller,when a level of the WDM signal light received by the first opticalamplifier changes by Δ, controls a level of the WDM signal lightreceived by the second optical amplifier to change by approximately −Δ.

[0019] Objects of the present invention are further achieved byproviding an apparatus which includes optical amplifiers opticallyconnected together, each optical amplifier having a corresponding gain.A controller causes the sum of the gains of the optical amplifiers to beconstant. There can be two or more optical amplifiers opticallyconnected together in this manner.

[0020] In addition, objects of the present invention are achieved byproviding an apparatus comprising first and second optical amplifiersconnected together, and a controller causing the sum of the gains of thefirst and second optical amplifiers to be constant. The controller caninclude a variable attenuator optically connected between the first andsecond optical amplifiers, where the attenuation of the variableattenuator is controlled to cause the sum of the gains of the first andsecond optical amplifiers to be constant.

[0021] Further, objects of the present invention are achieved byproviding an apparatus which includes first and second opticalamplifiers, and a gain adjustor. The first optical amplifier amplifies alight with a gain of the first optical amplifier. The second opticalamplifier receives the light amplified by the first optical amplifier,and amplifies the received light with a gain of the second opticalamplifier. The gain adjustor detects a deviation in gain of the firstoptical amplifier from a target gain, and adjusts the gain of the secondoptical amplifier to compensate for the detected deviation.

[0022] Objects of the present invention are also achieved by providingan apparatus including a first optical amplifier which amplifies a lightwith a gain of the first optical amplifier. A first gain controllercontrols the gain of the first optical amplifier to be constant at atarget gain. A second optical amplifier receives the light amplified bythe first optical amplifier, and amplifies the received light with again of the second optical amplifier. A gain deviation detector detectsa deviation in gain of the first optical amplifier from the target gain.A gain adjustor adjusts the gain of the second optical amplifier tocompensate for the detected deviation in gain of the first opticalamplifier. A level controller can control a level of the light amplifiedby the first optical amplifier before being amplified by the secondoptical amplifier to be at a target level, wherein the level controlleradjusts the target level to compensate for a detected deviation in gainof the first optical amplifier by the gain deviation detector.

[0023] Moreover, objects of the present invention are achieved byproviding an optical amplifying device which includes first and secondoptical amplifiers. The first optical amplifier amplifies a light with again of the first optical amplifier. A first gain controller controlsthe gain of the first optical amplifier to be constant at a target gainfor the first optical amplifier. The second optical amplifier receivesthe light amplified by the first optical amplifier, and amplifies thereceived light with a gain of the second optical amplifier. A secondgain controller controls the gain of the second optical amplifier to beconstant at a target gain for the second optical amplifier. A gaindeviation detector detects a deviation in gain of the first opticalamplifier from the target gain of the first optical amplifier. A gainadjustor adjusts the target gain of the second optical amplifier tocompensate for the detected deviation in gain of the first opticalamplifier.

[0024] Objects of the present invention are also achieved by providingan apparatus including first and second optical amplifiers and a gainadjustor. The first optical amplifier amplifies a light with a gain ofthe first optical amplifier. The second optical amplifier receives thelight amplified by the first optical amplifier, and amplifies thereceived light with a gain of the second optical amplifier. The gainadjustor detects a deviation in gain of one of the first and secondoptical amplifiers from a target gain, and adjusts the gain of the otherof the first and second optical amplifiers to compensate for thedetected deviation.

[0025] In addition, objects of the present invention are achieved byproviding an apparatus which includes a first optical amplifieramplifying a light with a gain of the first optical amplifier. A firstgain controller controls the gain of the first optical amplifier to beconstant at a target gain for the first optical amplifier. A secondoptical amplifier receives the light amplified by the first opticalamplifier, and amplifies the received light with a gain of the secondoptical amplifier. A second gain controller controls the gain of thesecond optical amplifier to be constant at a target gain for the secondoptical amplifier. A gain adjustor detects a deviation in gain of one ofthe first and second optical amplifiers from its target gain, andadjusts the gain of the other of the first and second optical amplifiersto compensate for the detected deviation.

[0026] Objects of the present invention are achieved by providing anapparatus which includes a plurality of optical amplifiers cascadedtogether so that a light is amplified by each optical amplifier as thelight travels through the cascaded plurality of optical amplifiers, eachoptical amplifier amplifying the light with a corresponding gain. A gainadjustor detects a deviation in gain of one of the plurality of opticalamplifiers from a target gain, and adjusts the gain of at least one ofthe other of the plurality of optical amplifiers to compensate for thedetected deviation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] These and other objects and advantages of the invention willbecome apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

[0028]FIG. 1 is a diagram illustrating an optical fiber communicationsystem, according to an embodiment of the present invention.

[0029]FIG. 2 is a graph illustrating a gain tilt occurring in an opticalamplifier.

[0030]FIG. 3 (prior art) is a diagram illustrating a conventionaloptical amplification device.

[0031]FIG. 4 is a power diagram for the optical amplification device inFIG. 3.

[0032]FIG. 5 is a diagram illustrating an optical amplification device,according to an embodiment of the present invention.

[0033]FIG. 6 is a graph illustrating power levels for the opticalamplification device in FIG. 5, according to an embodiment of thepresent invention.

[0034]FIG. 7 is a diagram illustrating details of the opticalamplification device in FIG. 5, according to an embodiment of thepresent invention.

[0035]FIG. 8 is a diagram illustrating an optical amplification device,according to an additional embodiment of the present invention.

[0036]FIG. 9 is a diagram illustrating details of the opticalamplification device in FIG. 8, according to an embodiment of thepresent invention.

[0037]FIG. 10 is a diagram illustrating details of the opticalamplification device in FIG. 8, according to an additional embodiment ofthe present invention.

[0038]FIG. 11 is a graph illustrating a change in a wavelengthcharacteristic of gain with a change in population inversion coefficientfor an EDF.

[0039]FIG. 12 is a diagram illustrating details of the opticalamplification device in FIG. 8, according to a further embodiment of thepresent invention.

[0040]FIG. 13 is a diagram illustrating an optical amplification device,according to a further embodiment of the present invention.

[0041] FIGS. 14(A) and 14(B) are diagrams illustrating differentoperational modes of the optical amplification device in FIG. 13,according to an embodiment of the present invention.

[0042]FIG. 15 is a diagram illustrating an optical amplification device,according to an additional embodiment of the present invention.

[0043]FIG. 16 is a diagram illustrating an optical amplification device.

[0044]FIG. 17 is a graph illustrating a gain-wavelength characteristicof an EDF at a set gain.

[0045]FIG. 18 is a graph illustrating a transmission characteristic of again equalizer corresponding to the gain-wavelength characteristic shownin FIG. 17.

[0046]FIG. 19 is a graph illustrating changes in power level of WDMsignal light per channel propagating in the optical amplification devicein FIG. 16.

[0047]FIG. 20 is a graph illustrating an example of the gain-wavelengthcharacteristic of an EDF when the gain changes.

[0048]FIG. 21 is a diagram illustrating an optical amplification devicefor use in a WDM optical communication system, according to anembodiment of the present invention.

[0049]FIG. 22 is a graph illustrating changes in power level of WDMsignal light per channel propagating in the optical amplification devicein FIG. 21, according to an embodiment of the present invention.

[0050]FIG. 23 is a graph illustrating a relation between input lightlevel to an EDF and gain of the EDF in the optical amplification devicein FIG. 21, according to an embodiment of the present invention.

[0051]FIG. 24 is a diagram illustrating an optical communication system,according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings, wherein like reference numerals refer tolike elements throughout.

[0053]FIG. 1 is a diagram illustrating an optical fiber communicationsystem, according to an embodiment of the present invention. Referringnow to FIG. 1, the system includes a first terminal station 102, asecond terminal station 104, an optical fiber transmission line 106connecting terminal stations 102 and 104, and a plurality of opticalrepeaters 108 arranged along optical fiber transmission line 106. Eachoptical repeater 108 includes an optical amplifier 110 opticallyconnected to optical fiber transmission line 106. Although FIG. 1 showstwo optical repeaters 108, more than two optical repeaters can be used,depending on the system design parameters. Further, in some systems, asingle optical repeater may be used.

[0054] First terminal station 102 includes a plurality of opticaltransmitters (TX) 112 (#1 to #N) respectively outputting a plurality ofoptical signals having different wavelengths. An optical multiplexer(MUX) 114 wavelength division multiplexes the optical signals outputfrom optical transmitters 112 (#1 to #N) to obtain WDM signal light. Anoptical amplifier (postamplifier) 116 amplifies the WDM signal lightobtained from optical multiplexer 114 to output an amplified WDM signallight to optical fiber transmission line 106.

[0055] Second terminal station 104 includes an optical amplifier(preamplifier) 118 amplifying the WDM signal light from optical fibertransmission line 106. An optical demultiplexer (DMUX) 120 demultiplexesthe amplified WDM signal light output from optical amplifier 118 into aplurality of optical signals. A plurality of optical receivers (RX) 122(#1 to #N) respectively receive the optical signals from opticaldemultiplexer 120.

[0056] With this configuration, loss of the WDM signal light can becompensated by at least one optical repeater 108 arranged along opticalfiber transmission line 106, thereby allowing long-haul transmission.Furthermore, a plurality of channels are transmitted by optical fibertransmission line 106, thereby increasing a transmission capacity.

[0057]FIG. 2 is a graph illustrating gain tilt occurring in an opticalamplifier. More specifically, FIG. 2 illustrates the spectra of outputlight when WDM signal light based on optical signals of four channels(wavelengths of 1548, 1551, 1554, and 1557 nm, respectively) having thesame power (−35 dBm/ch) is input into an EDFA (erbium doped fiberamplifier). In FIG. 2, the vertical axis represents output power (dBm),and the horizontal axis represents wavelength (nm).

[0058] The spectrum shown by A corresponds to the case where the powerof pump light is relatively high, causing a negative gain tilt in a bandof about 1.54 to 1. 56 μm. That is, the negative gain tilt is a gaintilt such that the gain decreases with an increase in wavelength, andthe derivative of gain (G) with respect to wavelength (λ) is negative(dG/dλ<0).

[0059] The spectrum shown by C corresponds to the case where the powerof pump light is relatively low, causing a positive gain tilt in a bandof about 1.54 to 1.56 μm. That is, the positive gain tilt is a gain tiltsuch that the gain increases with an increase in wavelength, and thederivative of gain (G) with respect to wavelength (λ) is positive(dG/dλ>0).

[0060] The spectrum shown by B corresponds to the case where the powerof pump light is optimum so that no gain tilt is induced or the gaintilt becomes flat in a band of about 1.54 to 1.56 μm, and the derivativeof gain (G) with respect to wavelength (μ) is zero (dG/dμ=0).

[0061] Each spectrum has such a shape that four sharp spectracorresponding to the optical signals of the four channels aresuperimposed on a gentle spectrum of ASE (amplified spontaneousemission). It is known that the wavelength characteristic of gain for asmall signal depends on an ASE spectrum.

[0062] In the case that a plurality of optical amplifiers are cascadedas shown in FIG. 1, a gain tilt occurring in each optical amplifier isaccumulated over the optical fiber transmission line, causing adegradation in signal-to-noise ratio in a low-level channel or adegradation in waveform due to nonlinear effects or the like in ahigh-level channel, so that a transmission distance for obtaining arequired receiving sensitivity is limited. Accordingly, in this kind ofsystem, it is greatly effective in increasing a transmission distance toperform a control such that the gain tilt in each optical amplifierbecomes flat. Further, the output level of each optical amplifier has anoptimum range. Accordingly, by controlling the output level of eachoptical amplifier so that it always falls within the optimum rangeirrespective of the input level of each optical amplifier, the inputdynamic range can be widened.

[0063]FIG. 3 is a diagram of a conventional optical amplification devicefor use as each of optical amplifiers 110, 116 and 118 in FIG. 1.Referring now to FIG. 3, a first-stage optical amplifier 6′ and asecond-stage optical amplifier 8′ are cascaded between an input port 2and an output port 4, and a variable optical attenuator (ATT) 10′ isoptically connected between optical amplifiers 6′ and 8′. Opticalamplifier 6′ is provided with a feedback loop 12 for automatic gaincontrol (AGC), and optical amplifier 8′ is provided with a feedback loop14 for AGC. Variable optical attenuator 10′ is included in a feedbackloop 16 for automatic level control (ALC) to maintain a constant outputlevel of the optical amplification device.

[0064] In FIG. 3, AGC is performed in each of optical amplifiers 6′ and8′, so that the wavelength characteristic of gain in each of opticalamplifiers 6′ and 8′ can be maintained constant. Furthermore, sincefeedback loop 16 for ALC is provided independently of feedback loops 12and 14 for AGC, a wide input dynamic range can be obtained.

[0065] However, with the optical amplification device in FIG. 3, thereis a problem that an excessive pump light power may be required in thecase that first-stage optical amplifier 6′ includes an erbium dopedfiber (EDF) and a pumping light source for supplying pump light to theEDF. This problem will now be more specifically described.

[0066]FIG. 4 is a power diagram of the optical amplification device inFIG. 3. In FIG. 4, the vertical axis represents optical power (dBm), andthe horizontal axis represents position on an optical path extendingfrom input port 2 to output port 4. Assuming that the input level offirst-stage optical amplifier 6′ increases by Δ, the output level ofoptical amplifier 6′ also increases by Δ because the gain G1 of theoptical amplifier 6′ is maintained constant irrespective of the inputlevel by feedback loop 12. The gain G2 of second-stage optical amplifier8′ is also maintained constant by feedback loop 14. Accordingly, theattenuation to be given by variable optical attenuator 10′ is increasedby the operation of feedback loop 16 for ALC so that the output level ofsecond-stage optical amplifier 8′ is maintained constant.

[0067] In general, high-power pump light is required to obtain a highoutput level of an EDFA. Accordingly, in the case of using an EDFA asfirst-stage optical amplifier 6′, high-power pump light is required toaccept a high input level. That is, with the optical amplificationdevice in FIG. 3, high-power pump light may be required to obtain a wideinput dynamic range.

[0068]FIG. 5 is a diagram illustrating an optical amplification device,according to an embodiment of the present invention. Referring now toFIG. 5, a first-stage optical amplifier 6 and a second-stage opticalamplifier 8 are cascaded between an input port 2 and an output port 4,and a variable optical attenuator 10 is optically connected betweenoptical amplifiers 6 and 8. Variable optical attenuator 10 variablyattenuates the transmitted light in accordance with a control signal CSsupplied thereto. Signal light (such as WDM signal light) supplied toinput port 2 and to be amplified is sequentially subjected toamplification by optical amplifier 6, attenuation by variable opticalattenuator 10, and amplification by optical amplifier 8, and is finallyoutput from output port 4. Optical amplifier 6 is additionally providedwith a feedback loop 18 for ALC. Feedback loop 18 serves to controloptical amplifier 6 so that the output level of optical amplifier 6 ismaintained constant. Similarly, optical amplifier 8 is additionallyprovided with a feedback loop 20 for ALC. Feedback loop 20 serves tocontrol optical amplifier 8 so that the output level of opticalamplifier 8 is maintained constant.

[0069] In this embodiment, the input level of first-stage opticalamplifier 6 is detected, and the control signal CS is generated from acontrol unit 22 so that, when the input level of optical amplifier 6changes by Δ (dBm in unit), the input level of optical amplifier 8changes by −Δ.

[0070] Therefore, in FIG. 5, control unit 22 and variable opticalattenuator 10 together operate as a controller which, when a level ofthe light received by optical amplifier 6 changes by Δ, controls a levelof the light received by optical amplifier 8 to change by approximately−Δ.

[0071] Optical amplifiers 6 and 8 each may include an optical amplifyingmedium and a pumping light source. The pumping light source providespump light to the optical amplifying medium. Signal light (such as WDMsignal light) is then amplified as it travels through the opticalamplifying medium, as long as the optical amplifying medium and the pumplight provide a gain band which includes a wavelength of the signallight. The term of “gain band” is defined as a band in which the opticalamplifying medium can generate a gain.

[0072] In the case that a doped fiber doped with a dopant including arare earth element is used as the optical amplifying medium, the dopedfiber can be pumped by supplying pump light having a predeterminedwavelength to the doped fiber. In this case, the gain generated in theoptical amplifying medium or the output level of the optical amplifyingmedium depends on the power of the pump light, so that a pumping lightsource may be included in the feedback loop for ALC.

[0073] In the case that an EDF is used as the optical amplifying medium,a laser diode oscillating in a 0.98 μm band (0.96 to 1.0 μm) or in a1.48 μm band (1.46 to 1.50 μm) may be used as the pumping light sourceto thereby obtain a gain band including a 1.55 μm band (1.50 to 1.60μm).

[0074] In the case that a semiconductor chip obtained by reducing thereflectance of opposite end faces of a laser diode is used as theoptical amplifying medium, the pumping can be performed by injecting acurrent into the semiconductor chip. In this case, the gain generated inthe semiconductor chip or the optical output level of the semiconductorchip depends on the injected current, so that a drive circuit for thesemiconductor chip may be included in the feedback loop for ALC.

[0075]FIG. 6 is a graph illustrating power levels in the opticalamplification device of FIG. 5, according to an embodiment of thepresent invention. In FIG. 6, the vertical axis represents optical power(dBm), and the horizontal axis represents position on an optical pathextending from input port 2 to output port 4.

[0076] Referring now to FIGS. 5 and 6, assuming that the input level offirst-stage optical amplifier 6 increases by Δ (dBm in unit), the gaingenerated in optical amplifier 6 decreases from G1 (dB in unit) to G1′(dB in unit) because the output level of optical amplifier 6 ismaintained constant by the feedback loop 18 for ALC. The attenuation ofvariable optical attenuator 10 is controlled by control unit 22, so thatthe input level of second-stage optical amplifier 8 changes by −Δ. Whenthe input level of optical amplifier 8 thus decreases by Δ, the gaingenerated in optical amplifier 8 increases from G2 (dB in unit) to G2′(dB in unit) because the output level of optical amplifier 8 ismaintained constant by feedback loop 20 for ALC.

[0077] Accordingly, the output level of second-stage optical amplifier 8is constant irrespective of the input level of first-stage opticalamplifier 6. Further, the sum of the gain generated in optical amplifier6 and the gain generated in optical amplifier 8 is constant irrespectiveof the input level of optical amplifier 6. That is, the relation ofG1+G2=G1′+G2′ is satisfied.

[0078] Therefore, as can be seen from FIG. 6, the total gain of aplurality of cascaded optical amplifiers is maintained constant.Although FIGS. 5 and 6 show only two optical amplifiers, the presentinvention is not intended to be limited in this manner. Instead, thepresent invention is applicable to a configuration of more than twooptical amplifiers cascaded together, so that the total gain of thecascaded optical amplifiers is maintained constant.

[0079] By maintaining constant the total sum of the gains generated in aplurality of cascaded optical amplifiers, the wavelength characteristicof gain can be maintained constant. This will now be described morespecifically in the case of cascading a plurality of doped fibers eachdoped with a dopant including a rare earth element.

[0080] In general, the wavelength characteristic G(λ) of the gain of onedoped fiber is expressed as follows:

G(λ)={tg*(λ)−(1−t)α(λ)}L

[0081] where g*(λ) (dB/m) is the emission spectrum, α (dB/m) is theabsorption spectrum, t is the population inversion coefficient(inversion parameter) of the doped fiber in its longitudinal direction,and L (m) is the length of the doped fiber.

[0082] Accordingly, in the case of cascading a plurality of doped fibershaving the same emission spectrum and the same absorption spectrum, thetotal gain G_(total)(λ) is given as follows: $\begin{matrix}{{G_{total}(\lambda)} = {\sum\limits_{k}{\left( {{t_{k}{g^{*}(\lambda)}} - {\left( {1 - t_{k}} \right){\alpha (\lambda)}}} \right\} L_{k}}}} \\{= {{\left( {\sum\limits_{k}{t_{k}L_{k}}} \right){g^{*}(\lambda)}} - {\left( {{\sum\limits_{k}L_{k}} - {\sum\limits_{k}{L_{k}t_{k}}}} \right){\alpha (\lambda)}}}} \\{= \left\{ {{\left( {\sum\limits_{k}{t_{k}{L_{k}/L_{total}}}} \right){g^{*}(\lambda)}} - {\left( {1 - {\sum\limits_{k}{L_{k}{t_{k}/L_{total}}}}} \right){\alpha (\lambda)}}} \right\}}\end{matrix}\quad$

[0083] where t_(k) and L_(k) are the population inversion coefficientand the length of the k-th doped fiber, respectively, and L_(total) isthe total length $\left( {\sum\limits_{k}L_{k}} \right)$

[0084] of the cascaded doped fiber.

[0085] Accordingly, by maintaining constant the total sum of the gainsgenerated in a plurality of cascaded optical amplifiers, the wavelengthcharacteristic of gain of the amplifier chain can be maintainedconstant.

[0086] Particularly, in FIG. 5, feedback loops 18 and 20 for ALC andcontrol unit 22 for variable optical attenuator 10 are used, so that thetotal sum of the gains generated in the optical path extending frominput port 2 to output port 4 can be maintained constant. Accordingly,by using an optical amplification device of the embodiments of thepresent invention as each of optical amplifiers 110, 116, and 118 inFIG. 1, accumulation of gain tilts can be prevented to thereby allow anincrease in transmission distance.

[0087] Further, since feedback loop 20 for ALC is provided forsecond-stage optical amplifier 8, the output level of second-stageoptical amplifier 8 can be maintained constant irrespective of the inputlevel of first-stage optical amplifier 6, thereby widening the inputdynamic range.

[0088] Further, feedback loop 18 for ALC is provided also forfirst-stage optical amplifier 6. Therefore, in the case that opticalamplifier 6 includes a doped fiber and a pumping light source, anundesirable increase in pump light power can be prevented, thus avoidingthe problem in the related art shown in FIG. 3.

[0089] In addition, to maximize the obtainable total gain in theconfiguration of FIG. 5, the attenuation of variable optical attenuator10 may be controlled to become a minimum when the input level offirst-stage optical amplifier 6 is a lower limit.

[0090]FIG. 7 is a diagram illustrating details of the opticalamplification device in FIG. 5, according to an embodiment of thepresent invention. Referring now to FIG. 7, EDFs 24 and 26 are adoptedas the optical amplifying medium of optical amplifiers 6 and 8,respectively, to obtain a gain band including a 1.55 μm band where alowest loss is obtained in a silica fiber. A first end 24A of EDF 24 isoptically connected through a WDM coupler 28 and an optical coupler 30to input port 2, and a second end 24B of EDF 24 is optically connectedthrough an optical coupler 32 to an input port of variable opticalattenuator 10. A first end 26A of EDF 26 is optically connected to anoutput port of variable optical attenuator 10, and a second end 26B ofEDF 26 is optically connected through a WDM coupler 34 and an opticalcoupler 36 to output port 4.

[0091] To prevent formation of an optical resonator structure includingeach of EDFs 24 and 26, one or more optical isolators may be provided inthe optical path between input port 2 and output port 4. With thisarrangement, the operation stability of the device can be improved.

[0092] Optical couplers 30 and 32 are used to detect the input level andthe output level of first-stage optical amplifier 6, respectively, andoptical coupler 36 is used to detect the output level of second-stageoptical amplifier 8. Accordingly, each of optical couplers 30, 32 and 36can be fabricated without especially considering the wavelengthdependence of branching ratio.

[0093] WDM coupler 28 is used to supply pump light from a laser diode(LD) 38 into EDF 24 from its first end 24A, and WDM coupler 34 is usedto supply pump light from a laser diode 40 into EDF 26 from its secondend 26B. The wavelength of each pump light is different from thewavelength of signal light to be amplified. Accordingly, each of WDMcouplers 28 and 34 is fabricated in consideration of the wavelengthdependence of branching ratio. The oscillation wavelength of each oflaser diodes 38 and 40 is set to fall within a 0.98 μm band or a 1.48 μmband, for example, to obtain a gain band including a 1.55 μm band.

[0094] A drive current (bias current) is supplied from a drive circuit42 to laser diode 38. Feedback loop 18 for ALC for optical amplifier 6includes a photodetector (PD) 44 such as a photodiode and an ALC circuit46. Monitor light branched by optical coupler 32 is supplied through anoptical bandpass filter 48 to photodetector 44. Photodetector 44 outputsan electrical signal having a voltage level (or current level)corresponding to the power of the received monitor light. ALC circuit 46receives the output signal from photodetector 44 and controls the drivecurrent to be supplied from drive circuit 42 to laser diode 38 so thatthe level of the received signal becomes constant.

[0095] The pass band of optical bandpass filter 48 is set so as toinclude the wavelength of the signal light amplified in EDF 24 and notto include the wavelength of the residual pump light from laser diode 38having not contributed to the optical amplification in EDF 24. Thissetting allows ALC such that the output level of the signal lightamplified in EDF 24 becomes constant.

[0096] Laser diode 40 as the pumping light source for second-stageoptical amplifier 8 is supplied with a drive current (bias current) froma drive circuit 49. Feedback loop 20 for ALC for second-stage opticalamplifier 8 includes a photodetector 50 and an ALC circuit 52.

[0097] Monitor light branched by optical coupler 36 is supplied throughan optical bandpass filter 54 to photodetector 50. Photodetector 50outputs an electrical signal having a voltage level (or current level)corresponding to the power of the received monitor light. ALC circuit 52controls the drive current to be supplied from drive circuit 49 to laserdiode 40 so that the level of the output signal from photodetector 50becomes constant. Thus, the ALC for second-stage optical amplifier 8 isachieved by feedback loop 20 as similarly to feedback loop 18.

[0098] Control unit 22 for generating the control signal CS includes aphotodetector 56 and a control circuit 58. Monitor light branched byoptical coupler 30 is supplied through an optical bandpass filter 60 tophotodetector 56. Photodetector 56 outputs an electrical signal having avoltage level (or current level) corresponding to the power of thereceived monitor light. Control circuit 58 detects a change in the inputlevel of the signal light to be supplied into EDF 24 and to be amplifiedtherein, according to the output signal from photodetector 56, andgenerates the control signal CS so that an opposite amount is given tothe input level of EDF 26 by variable optical attenuator 10.

[0099] The pass band of each of optical bandpass filters 54 and 60 isset so as to include the wavelength of the signal light to be amplified.In particular, the pass band of optical bandpass filter 54 is preferablyset so as not to include the wavelength of the pump light, therebyeliminating the influence of the pump light from laser diode 38.

[0100] According to this embodiment of the present invention, the totalgain generated in the optical path extending from input port 2 to outputport 4 can be maintained constant to thereby maintain the wavelengthcharacteristic of gain constant. Furthermore, the output level at outputport 4 can be maintained constant irrespective of the input level atinput port 2, thereby widening the input dynamic range of the device.

[0101] Moreover, ALC is performed so that the output level of the signallight amplified in EDF 24 becomes constant, thereby preventing anundesirable increase in the power of the pump light to be output fromlaser diode 38.

[0102] In this embodiment of the present invention, the signal light andthe pump light propagate in the same direction in EDF 24 of first-stageoptical amplifier 6. In other words, first-stage optical amplifier 6 isa forward pumping type optical amplifier. On the other hand, the signallight and the pump light propagate in opposite directions in EDF 26 ofsecond-stage optical amplifier 8. In other words, second-stage opticalamplifier 8 is a backward pumping type optical amplifier. As amodification, first-stage optical amplifier 6 may be configured as abackward pumping type optical amplifier, and second-stage opticalamplifier 8 may be configured as a forward pumping type opticalamplifier. Further, each of optical amplifiers 6 and 8 may be configuredas a bidirectional pumping type optical amplifier obtained by combiningforward pumping and backward pumping.

[0103]FIG. 8 is a diagram illustrating an optical amplification device,according to an additional embodiment of the present invention.Referring now to FIG. 8, a control unit 66 connected to gain monitors 62and 64 controls variable optical attenuator 10. By contrast, in theembodiment shown in FIG. 5, control unit 22 controls variable opticalattenuator 10 according to the input level of first-stage opticalamplifier 6.

[0104] In FIG. 8, gain monitor 62 detects a first gain generated infirst-stage optical amplifier 6, and gain monitor 64 detects a secondgain generated in second-stage optical amplifier 8. Control unit 66generates a control signal CS so that the sum of the first and secondgains detected by gain monitors 62 and 64 becomes constant, and thecontrol signal CS is supplied to variable optical attenuator 10. Controlsignal CS controls the variable attenuation of variable opticalattenuator 10 to appropriately attenuate the light signal travellingfrom optical amplifier 6 to optical amplifier 8.

[0105] Therefore, in FIG. 8, control unit 66 and variable opticalattenuator 10 together operate as a controller which causes the sum ofthe gains of optical amplifiers 6 and 8 to be constant.

[0106] A power diagram as in FIG. 6 can be obtained for the opticalamplification device illustrated in FIG. 8. Accordingly, the wavelengthcharacteristic of gain can be maintained constant, and a wide inputdynamic range can be obtained. Further, the power of pump light can besuppressed.

[0107] With the optical amplification device of FIG. 8, it is apparentthat the power diagram satisfying the conditions of the presentinvention can be obtained even if any one of feedback loops 18 and 20 isomitted. Accordingly, any one of feedback loops 18 and 20 may beomitted. Further, both feedback loops 18 and 20 may be omitted. In thecase that each of optical amplifiers 6 and 8 includes an EDF and apumping light source for supplying pump light to the EDF under thecondition where none of feedback loops 18 and 20 are used, it isdesirable to add to at least one of optical amplifiers 6 and 8 an APC(automatic power control) loop for maintaining the power of the pumplight constant or an ACC (automatic current control) loop formaintaining a drive current for the pumping light source constant. Theaddition of APC or ACC is particularly effective to first-stage opticalamplifier 6, so as to achieve many of the objects of the presentinvention.

[0108]FIG. 9 is a diagram illustrating details of the opticalamplification device in FIG. 8, according to an embodiment of thepresent invention. Referring now to FIG. 9, the input level and theoutput level of first-stage optical amplifier 6 are reflected by outputelectrical signals from photodetectors 56 and 44, respectively.Accordingly, the gain generated in first-stage optical amplifier 6 canbe calculated by a gain calculating circuit 68 according to the ratio ordifference in level between the output electrical signals fromphotodetectors 56 and 44.

[0109] To detect the input level of second-stage optical amplifier 8, anoptical coupler 69, an optical bandpass filter 70, and a photodetector72 are provided as respectively corresponding to optical coupler 30,optical bandpass filter 60, and photodetector 56 for first-stage opticalamplifier 6. The gain generated in second-stage optical amplifier 8 canbe calculated by a gain calculating circuit 74 according to the ratio ordifference in level between output electrical signals fromphotodetectors 72 and 50.

[0110] A control circuit 76 (corresponding to control unit 66 in FIG. 8)generates a control signal CS so that the sum of the gains obtained fromgain calculating circuits 68 and 74 becomes constant and adjusts theattenuation of variable optical attenuator 10 according to the controlsignal CS.

[0111] The pass band of each of optical bandpass filters 48, 54, 60 and70 is set so as to include the wavelength of the signal light to beamplified. For example, in the case that the signal light to beamplified is WDM signal light, the pass band of each filter may be setso as to include the wavelength of an optical signal in one channel ormay be set so as to include the wavelengths of optical signals in pluralchannels. Alternatively, the pass band may be set so as to pass ASE in acertain band not including the wavelength of the signal light, becausethe power of ASE included in this band reflects the gain.

[0112] In any case, the pass band of each of optical bandpass filters48, 54 and 70 is desirably set so as not to include the wavelength ofthe pump light, so as to avoid the influence of the pump light.

[0113]FIG. 10 is a diagram illustrating details of the opticalamplification device in FIG. 8, according to an additional embodiment ofthe present invention. Referring now to FIG. 10, modified gaincalculating circuits 68′ and 74′ are used to detect the gains offirst-stage and second-stage optical amplifiers 6 and 8 according to theabsorptance of the pump light in EDFs 24 and 26, respectively.

[0114] Of the pump light supplied from laser diode 38 to EDF 24, theresidual pump light not absorbed by EDF 24 is branched from a mainoptical path (an optical path between input port 2 and output port 4) bya WDM coupler 78 provided between optical coupler 32 and variableoptical attenuator 10. The residual pump light branched by WDM coupler78 is supplied to a photodetector 80. Photodetector 80 outputs anelectrical signal having a voltage level (or current level)corresponding to the power of the residual pump light received.

[0115] The output signal from photodetector 80 reflects the power of theresidual pump light. The power of the pump light supplied from laserdiode 38 to EDF 24 reflects the drive current supplied from drivecircuit 42 to laser diode 38. Accordingly, the absorptance of the pumplight in EDF 24 can be obtained according to the output signal fromphotodetector 80 and a signal from drive circuit 42. Gain calculatingcircuit 68′ calculates the gain generated in EDF 24 according to theabsorptance of the pump light obtained. To detect the power of theresidual pump light in EDF 26, a WDM coupler 82 and a photodetector 84are provided as respectively corresponding to WDM coupler 78 andphotodetector 80. WDM coupler 82 is optically connected between EDF 26and variable optical attenuator 10.

[0116] Gain calculating circuit 74′ can detect the gain generated in EDF26 according to an output signal from photodetector 84 and a signal fromdrive circuit 49. Control circuit 76 generates a control signal CS sothat the sum of the two gains calculated by gain calculating circuits68′ and 74′ becomes constant, thereby maintaining constant thewavelength characteristic of total gain generated in the main opticalpath.

[0117]FIG. 11 is a graph illustrating a change in the wavelengthcharacteristic of gain generated in an EDF with a change in populationinversion coefficient (inversion parameter). In FIG. 11, the verticalaxis represents gain (dB) or local gain (dB/m), and the horizontal axisrepresents wavelength (nm). The wavelength characteristic of gaincontinuously changes from a characteristic shown by reference numeral 86toward a characteristic shown by reference numeral 88 with an increasein population inversion coefficient from 0 toward 1. The characteristic86 corresponding to the case where the population inversion coefficientis 0 provides a so-called absorption cross section, whereas thecharacteristic 88 corresponding to the case where the populationinversion coefficient is 1 provides a so-called emission cross section.Accordingly, the gain at a certain wavelength decreases with an increasein absorptance of pump light in an EDF. Thus, the gain generated in anEDF and the absorptance of pump light in the EDF are in a 1:1correspondence, so that the gain can be calculated according to theabsorptance of pump light.

[0118]FIG. 12 is a diagram illustrating details of the opticalamplification device in FIG. 8, according to a further embodiment of thepresent invention. Referring now to FIG. 12, the gains generated in EDFs24 and 26 are detected in accordance with the fact that the gaingenerated in an EDF is reflected by the power of spontaneous emissionlight (SE light) emitted sideward from the EDF. To detect the power ofspontaneous emission light emitted sideward from EDF 24 over its entirelength or at a part thereof, a photodetector 90 is provided in thevicinity of EDF 24. Similarly, to detect the power of spontaneousemission light emitted sideward from EDF 26 over its entire length or ata part thereof, a photodetector 92 is provided in the vicinity of EDF26.

[0119] Gain calculating circuits 68″ and 74″ calculate the gainsgenerated in EDFs 24 and 26 according to output signals fromphotodetectors 90 and 92, respectively. Accordingly, control circuit 76can generate a control signal CS according to output signals from gaincalculating circuits 68″ and 74″ so that the sum of the gains generatedin EDFs 24 and 26 becomes constant.

[0120] The attenuation of the variable optical attenuator 10 isdetermined according to the control signal CS, thereby maintainingconstant the total sum of the gains generated in the optical pathbetween input port 2 and output port 4. Accordingly, the wavelengthcharacteristic of gain of the device can be maintained constant, and awide input dynamic range can be obtained. Further, the power of pumplight can be suppressed.

[0121] In FIG. 12, photodetectors 90 and 92 are used to respectivelyreceive the spontaneous emission light emitted sideward from EDFs 24 and26. Alternatively, EDFs 24 and 26 may be wound to form the same loop,and a single photodetector may be provided in the vicinity of the loopof EDFs 24 and 26 to detect an optical power corresponding to the sum ofthe powers of the simultaneous emission light emitted sideward from EDFs24 and 26. This optical power reflects the sum of the gains generated inEDFs 24 and 26. Accordingly, control circuit 76 may generate a controlsignal CS according to an output signal from the single photodetector,thereby maintaining constant the total sum of the gains generated in theoptical path between input port 2 and output port 4. In this case, oneof photodetectors 90 and 92 and one of gain calculating circuits 68″ and74″ can be omitted, thereby simplifying the configuration of the device.

[0122]FIG. 13 is a diagram illustrating an optical amplification device,according to a further embodiment of the present invention. Morespecifically, FIG. 13 illustrates the configuration of a wideband EDFA.Referring now to FIG. 13, an EDF 200 and a pumping light source 210represent a low-noise preamplifier stage. An automatic gain control(AGC) circuit 220 controls pumping light source 210 to provide automaticgain control. A variable attenuator (VAT) 230 is controlled by anautomatic level control (ALC) circuit 240 to appropriately attenuate theoutput of the preamplifier stage. EDFs 242 and 244 are pumped by pumpinglight sources 246 and 248, respectively, to provide a post amplifierstage with high output power. An automatic gain control (AGC) circuit250 controls pumping light sources 246 and 248 to provide automatic gaincontrol. Therefore, AGC is employed in both the preamplifier stage andthe post amplifier stage.

[0123] Gain equalizers (GEQ) 252 and 254 are utilized to flatten thegain in the entire signal band for the pre-amplifier stage and thepost-amplifier stage, respectively. A dispersion compensating fiber(DCF) 256 can be positioned along the transmission line.

[0124] A supervisory (SV) circuit extracts supervisory information froma supervisory channel at a supervisory wavelength λ_(sv), and uses theextracted information to control ALC circuit 240.

[0125] FIGS. 14(A) and 14(B) are diagrams illustrating differentoperational modes of the optical amplification device in FIG. 13,according to an embodiment of the present invention. Referring now toFIG. 14(A), with this operational mode, the preamplifier stage employsautomatic power control (APC) provided by AGC circuit 220. As shown inFIG. 14(A), the decreasing amount of the preamplifier stage gain ΔG isadded to the gain of the postamplifier stage by controlling variableattenuator 230 to keep the total gain constant.

[0126] As shown in FIG. 14(B), with this operational mode, thepreamplifier stage employs automatic level control (ALC). As shown inFIG. 14(B), the decreasing amount of the preamplifier stage gain ΔG isadded to the gain of the postamplifier stage by controlling variableattenuator 230 to keep the total gain constant.

[0127]FIG. 15 is a diagram illustrating an optical amplification deviceaccording to an additional embodiment of the present invention.

[0128] Various of the above embodiments of the present invention relateto two optical amplifiers cascaded together. However, the presentinvention is applicable to configurations where three or more opticalamplifiers are cascaded together.

[0129] Moreover, according to the above embodiments of the presentinvention, an optical attenuator is positioned between opticalamplifiers. However, the present invention is applicable toconfigurations where more than one optical attenuator is positionedbetween optical amplifiers, and to configurations with more than twooptical amplifiers cascaded together, and optical attenuators positionedat various locations amongst the cascaded optical amplifiers.

[0130] According to the above embodiments of the present invention, itis possible to provide a method and device for optical amplification anda novel system including the device which can maintain the wavelengthcharacteristic of gain constant, can obtain a wide input dynamic range,and can suppress the power of pump light.

[0131] According to the above embodiments of the present invention, avariable optical attenuator is positioned between first and secondoptical amplifiers. The first and second optical amplifiers can each beprovided with an ALC feedback loop, for maintain output levels at aconstant value. The input level of the first optical amplifier isdetected, and the variable optical attenuator is controlled so that,when the input level of the first optical amplifier changes by Δ (dBm inunit), the input level of the second optical amplifier changes byapproximately −Δ.

[0132] Therefore, as indicated above, according to various embodimentsof the present invention, when the input level of a first opticalamplifier changes by Δ, the input level of a second optical amplifierchanges by approximately −Δ. For example, to change by approximately −Δ,the input level of the second optical amplifier should preferably changeby −(Δ±Δ/20). However, a change of approximately −Δis not intended to belimited to this preferable range, and other ranges may be sufficient toprovide sufficient operation.

[0133] According to the above embodiments of the present invention, avariable optical attenuator is positioned between first and secondoptical amplifiers. The variable optical attenuation provides a variableattenuation in accordance with a control signal supplied thereto. Afirst gain monitor detects the gain of the first optical amplifier and asecond gain monitor detects the gain of the second optical amplifier.The variable optical attenuator is controlled so that the sum of thedetected gains of the first and second optical amplifiers becomesconstant.

[0134] Various optical amplification devices for use in WDM opticalcommunication systems are described above. Japanese Patent Laid-openNos. Hei. 8-248455 and Hei. 10-51057, filed by the Applicants, discloseadditional optical amplification devices for use in WDM opticalcommunication systems. In these optical amplification devices, WDMsignal light obtained by multiplexing a plurality of optical signalshaving different wavelengths is subjected to batch amplification bytwo-stage amplifier sections each employing, for example, an erbiumdoped fiber (EDF). In the batch amplification, AGC is performed tocontrol the gain in each of the front-stage and rear-stage amplifiersections to be constant. By this control, the wavelength dependence ofgain (which will be hereinafter referred to as gain-wavelengthcharacteristic) of each EDFA can be maintained constant even when inputlight power changes. By effectively compensating for the gain-wavelengthcharacteristic of the optical amplifier, WDM signal light whose channelsare equalized in level can be obtained. Further, there has been proposedanother optical amplification device which can obtain stable amplifyingcharacteristics in the case that the number of wavelengths (the numberof channels) included in WDM signal light is changed.

[0135] In such optical amplification devices, a large-capacity pumpingsource must be used to realize constant gain control. In general, thepower level of WDM signal light per wavelength (per channel) input tothe optical amplification device is not a fixed value, but is varied. Tocontrol the gain to be constant against such variations in input lightlevel, the power of pump light must be controlled substantiallyexponentially. Accordingly, it is necessary to provide a relativelylarge-capacity pumping source which can support such exponentialcontrol.

[0136] While the optical amplification device is required to have arequired input dynamic range supporting variations in input light level,it is also required to output WDM signal light whose channels areequalized in level. For example, this requirement is met by a proposedmethod for controlling an optical attenuation so that the output lightlevel per channel becomes constant by providing a variable opticalattenuator between the front-stage and rear-stage amplifier sections ofthe optical amplification device.

[0137] In this case, the gain of the front-stage amplifier section isset so that the level of output light from the front-stage amplifiersection becomes a constant value or more when the input light level isminimum., Accordingly, also when the input light level is high, theamplification with the above set gain is carried out and thereafter theamplified signal light is attenuated to a given level by the variableoptical attenuator. Therefore, when the input light level is high,excess optical amplification is performed in the front-stage amplifiersection to realize AGC. Providing a costly high-capacity pumping sourceto allow such excess optical amplification is a disadvantage from theviewpoint of cost reduction of the optical amplification device.

[0138] Further, in an optical amplification device (such as an opticalamplifier) having a two-stage amplifying configuration, it is effectiveto set the gain of the front-stage amplifier section to a high value inreducing a noise figure (NF). However, realization of this setting alsorequires providing a large-capacity pumping source. That is, setting thegain to a high value in the case of a low input light level can berelatively easily realized also by using a normal-capacity pumpingsource. However, when the input light level is increased under AGC withthe gain being set to a high value, the power of pump light by thenormal-capacity pumping source becomes lacking to cause a possibilitythat the AGC does not effectively function. For this reason, thelarge-capacity pumping source is required.

[0139] Thus, such an optical amplification device applied to a WDMoptical communication system has a problem that an excess power of pumplight is required in the front-stage amplifier section, so as to ensurea required input dynamic range supporting variations in input lightlevel, to equalize the levels of all channels of output light, and toimprove noise characteristics.

[0140]FIG. 16 is a diagram illustrating an optical amplification deviceapplicable to a WDM optical transmission system.

[0141] Referring now to FIG. 16, the optical amplification device has atwo-stage amplification configuration including a front-stage amplifiersection 501 amplifying WDM signal light input to an input terminal Tin,a middle-stage ALC section 503 connected to an output end of front-stageamplifier section 501, and a rear-stage amplifier section 505 amplifyingWDM signal light passed through middle-stage ALC section 503 andoutputting amplified WDM signal light from an output terminal Tout.

[0142] In front-stage amplifier section 501, the WDM signal light inputto the input terminal Tin is supplied through a beam splitter 510, anoptical isolator 511, and a WDM coupler 512 to an erbium doped fiber(EDF) 513. Pump light is emitted from a pumping source (LD) 518 andsupplied through WDM coupler 512 to EDF 513 from its front end. Thus,EDF 513 is forward pumped by the pump light.

[0143] The drive condition of pumping source 518 is controlled accordingto a signal from an AGC circuit 520. More specifically, a part of theWDM signal light to be supplied to EDF 513 and a part of the amplifiedWDM signal light output from EDF 513 are respectively extracted by beamsplitters 510 and 516 and respectively converted into electrical signalsby photodetectors (PD) 517 and 519. These electrical signals fromphotodetectors 517 and 519 are input to AGC circuit 520. In AGC circuit520, an actual gain is obtained according to the input signals fromphotodetectors 517 and 519, and a signal for controlling the drivecondition of pumping source 518 so that the actual gain becomes constantis generated.

[0144] Thus, the gain by EDF 513 is controlled to be constant, and theWDM signal light is input into EDF 513 and passed therethrough, therebyamplifying the WDM signal light. The WDM signal light amplified by EDF513 is supplied through an optical isolator 514 to a gain equalizer(GEQ) 515. Gain equalizer 515 is an optical filter for compensating fora gain-wavelength characteristic of EDF 513. More specifically, in thecase that EDF 513 has a gain-wavelength characteristic at apredetermined gain as shown in FIG. 17, an optical filter having atransmittance-wavelength characteristic as shown by a solid line in FIG.18 may be used as gain equalizer 515. The WDM signal light passedthrough gain equalizer 515 is supplied through beam splitter 516 tomiddle-stage ALC section 503.

[0145] In middle ALC section 503, the WDM signal light from front-stageamplifier section 501 is supplied to a variable optical attenuator(VATT) 530 as a level adjusting device. An optical attenuation byvariable optical attenuator 530 is controlled according to a signal froman ALC circuit 534. More specifically, a part of the WDM signal lightoutput from variable optical attenuator 530 is extracted by a beamsplitter 531 and converted into an electrical signal by a photodetector533. This electrical signal is input to ALC circuit 534. In ALC circuit534, a signal for controlling the optical attenuation by variableoptical attenuator 530 so that the level of the WDM signal light perchannel to be output from variable optical attenuator 530 becomesconstant is generated according to a result of comparison between an ALCreference value (Valcref) supplied from a reference value generatingcircuit 535 and the signal supplied from photodetector 533.

[0146] Although not shown, information on the number of wavelengths (thenumber of channels) of the WDM signal light is given to reference valuegenerating circuit 535 from the outside thereof, and the ALC referencevalue is output so as to support a change in the number of channels.

[0147] The WDM signal light output from variable optical attenuator 530is supplied through a beam splitter 531 to a dispersion compensatingfiber (DCF) 532. Dispersion compensating fiber 532 functions tocompensate for a chromatic dispersion characteristic of an opticaltransmission line connected to the optical amplification device.However, in the case that no dispersion compensation is required,dispersion compensating fiber 532 may be omitted. The WDM signal lightpassed through dispersion compensating fiber 532 is supplied torear-stage amplifier section 505.

[0148] In rear-stage amplifier section 505, the WDM signal light frommiddle-stage ALC section 503 is supplied through a beam splitter 550, again equalizer 551, an optical isolator 552, and a WDM coupler 553 to anEDF 554. Similar to gain equalizer 515 in front-stage amplifier section501, gain equalizer 551 is an optical filter for compensating for again-wavelength characteristic of EDF 554 at a predetermined gain. Pumplight emitted from a pumping source 557 is supplied through WDM coupler553 to EDF 554 from its front end, thereby forward pumping EDF 554.

[0149] The drive condition of pumping source 557 is controlled accordingto a signal from an AGC circuit 559. More specifically, a part of theWDM signal light to be input to EDF 554 and a part of the amplified WDMsignal output from EDF 554 are respectively extracted by beam splitters550 and 555 and respectively converted into electrical signals byphotodetectors 556 and 558. These electrical signals from photodetectors556 and 558 are input to AGC circuit 559. In AGC circuit 559, an actualgain is obtained according to the input signals from photodetectors 556and 558, and a signal for controlling the drive condition of pumpingsource 557 so that the actual gain becomes constant is generated. Thus,the gain by EDF 554 is controlled to be constant, and the WDM signallight is input into EDF 554 and passed therethrough, thereby amplifyingthe WDM signal light. The WDM signal light amplified by EDF 554 ispassed through beam splitter 555 and output from the output terminalTout of this optical amplifier.

[0150] EDFs 513 and 554, pumping sources 518 and 557, and WDM couplers512 and 553 in front-stage and rear-stage amplifier sections 501 and 505function as an example of an optical amplifying device. Further, AGCcircuits 520 and 559, photodetectors 517, 519, 556, and 558, and beamsplitters 510, 516, 550, and 555 in front-stage and rear-stage amplifiersections 501 and 505 function as examples of a constant gain controldevices. Further, gain equalizers 515 and 551 in front-stage andrear-stage amplifier sections 501 and 505 function as examples ofgain-wavelength characteristic compensating devices. Further, ALCcircuit 534, beam splitter 531, photodetector 533, and reference valuegenerating circuit 535 in middle-stage ALC section 503 function asexamples of a constant level control device.

[0151] AGC is performed in each of front-stage amplifier section 501 andrear-stage amplifier section 505 according to detected levels of the WDMsignal light on the upstream and downstream sides of the correspondingEDF. As a modification, AGC may be performed by detecting amplifiedspontaneous emission (ASE) generated in each EDF and determining thegain by each EDF according to a detected level of the ASE. Further,while forward pumping is performed in each of front-stage amplifiersection 501 and rear-stage amplifier section 505, backward pumping orbidirectional pumping may be performed in each amplifier section.

[0152]FIG. 19 is a graph illustrating changes in power level of the WDMsignal light per channel propagating in the optical amplification deviceof FIG. 16. As shown in FIG. 19, although there is a change in inputlevel per channel input to the optical amplifier, the level of the WDMsignal light per channel is controlled to be constant in middle-stageALC section 503. Accordingly, the level of the WDM signal light perchannel amplified under the constant gain control in rear-stageamplifier section 505 is also maintained constant.

[0153] To ensure a required input dynamic range and maintain AGC forfront-stage amplifier section 501, pumping source 518 must be able tosupply pump light having a high power to EDF 513. In the case thatpumping source 518 cannot supply pump light having a power enough tomaintain a set gain for high-level input light as shown by a broken linein FIG. 19, the gain by EDF 13 becomes low to result in a change in thegain-wavelength characteristic shown in FIG. 17. If the gain-wavelengthcharacteristic of EDF 513 changes, the compensation by gain equalizer515 does not effectively function, and as a result, the level of the WDMsignal light per channel output from the optical amplifier cannot bemaintained constant.

[0154] A change in the gain-wavelength characteristic of an EDF will nowbe described in brief.

[0155]FIG. 20 is a graph illustrating an example of changes in thegain-wavelength characteristic of an EDF with changes in the gain of theEDF. In the example shown in FIG. 20, the gain-wavelength characteristichas a negative tilt such that the gain decreases with an increase inwavelength when the gain of the EDF is higher, whereas thegain-wavelength characteristic has a positive tilt such that the gainincreases with an increase in wavelength when the gain of the EDF islower. In this manner, it has been confirmed that the gain-wavelengthcharacteristic of an EDF is such that the gain tilt changes from apositive tilt to a negative tilt with an increase in the gain of theEDF, i.e., an increase in pump energy. Further, it has been reportedthat the gain tilt also changes according to an input light level or anEDF length (e.g., Y. Nakabayashi et al., “Flattening of multi-wavelengthbatch amplification of optical fiber amplifier using fiber amplificationfactor control”, ShingakuGiho, OCS94-66; S. Yoshida et al., “Wavelengthmultiplexed signal common amplification characteristics ofhigh-concentration A1 codoped EDFA”, ShingakuGiho, OCS95-9; Y, Sugaya etal., “A study of configuration method for wavelength multiplexingEr-doped fiber optical amplifier”, ShingakuGiho).

[0156] In FIG. 16, pumping source 518 having a relatively large capacityis required to maintain the constant gain control of front-stageamplifier section 501, causing an increase in cost of the opticalamplifier.

[0157] Noise characteristics of the optical amplification device in FIG.16 will now be described. The noise figure (NF) of the optical amplifieras a whole can be calculated in accordance with the following Equation(1).

[0158] Equation (1):

NF [dB]=LOSSf+10×log{10^(NFf/10)+(10^(LOSSm/10)+10^(NFr/10))/10^(Gf/10)}

[0159] where LOSSf is the loss on the upstream side of EDF 513, NFf isthe noise figure of EDF 513, LOSSm is the loss on the downstream side ofEDF 513 and on the upstream side of EDF 554 (the loss in the middleportion of the optical amplifier), NFr is the noise figure of EDF 554,and Gf is the gain of EDF 513. In the case that LOSSf=2 dB, NFf=4 dB,LOSSm=15 dB, NFr=6 dB, and Gf=15 dB, the noise figure of the opticalamplifier as a whole is calculated to NF=7.61 dB from Equation (1).

[0160] As apparent from Equation (1), the losses LOSSf and LOSSm or thenoise figures NFf and NFr must be decreased or the gain Gf must beincreased to reduce the noise figure NF of the optical amplifier as awhole. However, decreasing the losses LOSSf and LOSSm or the noisefigures NFf and NFr is limited because they depend on thecharacteristics of the respective devices. On the other hand, increasingthe gain Gf can be realized by increasing the power of the pump light tobe supplied to EDF 513. In particular, when the level of signal lightinput to the optical amplifier becomes low, a small value of the noisefigure NF of the optical amplifier must be ensured for the purpose ofincreasing an S/N ratio at a receiving end.

[0161] In the case that the level of input light is low, the noisefigure NF can be relatively easily reduced by increasing the gain Gf ofEDF 513, because not so high pump light power is required. However, thegain Gf of EDF 513 must be controlled to be constant because thegain-wavelength characteristic of EDF 513 must be compensated by gainequalizer 515. Accordingly, although the noise figure NF can be reducedby setting the gain Gf of EDF 513 to a high value in the case that thelevel of input light is low, there is a possibility that when the levelof input light becomes high, the pump light power may lack to cause aproblem that AGC is not effected and there occurs a difference in levelof output light between channels.

[0162] Consequently, as far as the optical amplification device in FIG.16 is required to have a required input dynamic range, the power of pumplight to be supplied to EDF 513 must be increased to increase the gainGf for the purpose of reducing the noise figure NF.

[0163] In these circumstances, according to embodiments of the presentinvention as will be discussed in more detail below, an opticalamplification device (such as an optical amplifier) for WDM has beenachieved by improving the configuration in FIG. 16 to thereby ensure arequired input dynamic range and allow effective compensation for thegain-wavelength characteristic of the EDF and a reduction in the noisefigure NF without restriction by AGC.

[0164]FIG. 21 is a diagram illustrating an optical amplification deviceaccording to an embodiment of the present invention. In FIG. 21,substantially the same parts as those of the optical amplificationdevice in FIG. 16 are denoted by the same reference numerals.

[0165] The optical amplification device shown in FIG. 21 has beenimproved over the configuration shown in FIG. 16 by detecting a changein gain of front-stage amplifier section 501, making the ALC referencevalue in middle-stage ALC section 503 changeable according to thedetected change in gain, and making the set gain in rear-stage amplifiersection 505 changeable according to the detected change in gain. Theseimprovements can eliminate the need for use of a large-capacity pumpingsource as pumping source 518 in front-stage amplifier section 501. Morespecifically, the configuration of the optical amplification device inFIG. 21 is different from the configuration in FIG. 16 in thatfront-stage amplifier section 501 further includes log transformers(LOG) 521 and 522 and subtracters (−) 523 and 524, in that middle-stageALC section 503 further includes a log transformer 536 and a subtracter537, and in that rear-stage amplifier section 505 further includes a logtransformer 560, an adder 561, and an anti-log transformer 562.

[0166] In this embodiment of the present invention, log transformers 521and 522 and subtracter 523 function as an example of a gain measuringdevice, and subtracter 524 functions as an example of a gain deviationcomputing device. Further, subtracter 537 functions as an example of areference level changing device, and log transformer 560, adder 561, andanti-log transformer 562 function as an example of a reference gainchanging device.

[0167] Input terminals of log transformers 521 and 522 in front-stageamplifier section 501 are connected to output terminals ofphotodetectors 517 and 519, respectively, so that log transformers 521and 522 respectively receive electrical signals from photodetectors 517and 519 and transform the voltage levels of the electrical signals intologarithmic values, which are in turn supplied to subtracter 523.Subtracter 523 subtracts the level of the output signal from logtransformer 521 from the level of the output signal from log transformer522 to obtain a voltage value Vagc corresponding to the gain of EDF 513,which is in turn supplied to subtracter 524. Subtracter 524 subtractsthe voltage value Vagc output from subtracter 523 from a predeterminedgain reference value Vagcref to obtain a gain correction value Vadj,which is in turn supplied to middle-stage ALC section 503 and rear-stageamplifier section 505.

[0168] An input terminal of log transformer 536 in middle-stage ALCsection 503 is connected to an output terminal of photodetector 533, sothat log transformer 536 receives an electrical signal fromphotodetector 533 and transforms a voltage level of the electricalsignal into a logarithmic value, which is in turn supplied to one of twoinput terminals of ALC circuit 534. Subtracter 537 receives the gaincorrection value Vadj from subtracter 524 in front-stage amplifiersection 501 and an ALC reference value Valcref from reference valuegenerating circuit 535, and subtracts the gain correction value Vadjfrom the ALC reference value Valcref to obtain a new ALC reference valueValcref', which is in turn supplied to the other input terminal of ALCcircuit 534. ALC circuit 534 controls the optical attenuation ofvariable optical attenuator 530 according to the new ALC reference valueValcref' so that the level of the WDM signal light per channel becomesconstant.

[0169] An input terminal of log transformer 560 in rear-stage amplifiersection 505 is connected to an output terminal of photodetector 556, sothat log transformer 560 receives an electrical signal fromphotodetector 556 and transforms a voltage level of the electricalsignal into a logarithmic value, which is in turn supplied to adder 561.Adder 561 adds the gain correction value Vadj from subtracter 524 infront-stage amplifier section 501 to the voltage value output from logtransformer 560 to obtain a sum, which is in turn supplied to anti-logtransformer 562. Anti-log transformer 562 transforms the logarithmicvoltage value output from adder 561 into an anti-logarithmic value,which is in turn supplied to one of two input terminals of AGC circuit559. AGC circuit 559 controls the gain of EDF 554 to be constantaccording to the signal from anti-log transformer 562 and the signalfrom photodetector 558.

[0170] Therefore, in FIG. 21, log transformer 521, log transformer 522,subtracter 523, subtracter 524, log transformer 560, adder 561 andanti-log transformer 562 together operate as a gain adjustor whichdetects a deviation in gain of front-stage amplifier section 501 from atarget (or reference) gain, and adjusts the gain of rear-stage amplifiersection 505 to compensate for the detected deviation. However, the useof such log transformers, subtracters and anti-log transformersrepresents only one embodiment of such a gain adjustor, andmodifications and other designs for a gain adjustor can easily beimplemented.

[0171] Moreover, in FIG. 21, log transformer 521, log transformer 522,subtracter 523, subtracter 524 and subtracter 537 together operate as alevel controller which controls a level of the light amplified byfront-stage amplifier section 501 before being amplified by rear-stageamplifier section 505 to be at a target level. The level controlleradjusts the target level to compensate for a detected deviation in gainof front-stage amplifier section 501 from a reference gain. However, theuse of such log transformers and subtracters represents only oneembodiment of such a level controller, and modifications and otherdesigns for a level adjuster can easily be implemented.

[0172] The operation of the optical amplification device in FIG. 21 willnow be described in more detail.

[0173]FIG. 22 is a graph illustrating changes in power level of the WDMsignal light per channel propagating in the optical amplification devicein FIG. 21, according to an embodiment of the present invention. In FIG.22, the changes in power level in the case of a low level of input lightas shown by a long and short dashed line are similar to those shown inFIG. 19. That is, the low-level WDM signal light input to front-stageamplifier section 501 is amplified with a sufficiently high gain Gf setin consideration of decreasing a noise figure, and at this time thegain-wavelength characteristic of EDF 513 is effectively compensated bygain equalizer 515. In this case, the gain correction value Vadj outputfrom subtracter 524 is zero because the AGC gain reference value Vagcrefis predetermined according to the gain Gf. The WDM signal light suppliedfrom front-stage amplifier section 501 to middle-stage ALC section 503is attenuated to a given level according to the ALC reference valueValcref because the gain correction value Vadj is zero. Thereafter, theWDM signal light is supplied through dispersion compensating fiber 532to rear-stage amplifier section 505. The WDM signal light input torear-stage amplifier section 505 is amplified with the predeterminedgain Gr, and at this time the gain-wavelength characteristic of EDF 554is effectively compensated by gain equalizer 551. Thus, in the case thatthe level of input light is low, the noise figure of the opticalamplifier as a whole can be reduced by setting the gain Gf offront-stage amplifier section 501 to a high value. Further, thegain-wavelength characteristics of EDFs 513 and 554 in front-stage andrear-stage amplifier sections 501 and 505 are effectively compensated bygain equalizers 515 and 551, respectively, because the pump light poweris relatively low and the constant gain control can be maintainedirrespective of a slight change in input light level. Therefore, the WDMsignal light whose channels are equalized in level can be output fromthe optical amplifier.

[0174] In the case that the level of input light is high as shown by asolid line in FIG. 22, pumping source 518 cannot supply pump lighthaving a high power enough to maintain the gain Gf constant, and the WDMsignal light is amplified with a gain Gf' lower than the gain Gf. Whenthe gain is reduced, the gain-wavelength characteristic of EDF 513changes so as to have a positive tilt as shown in FIG. 20, so that thecompensation of EDF 513 by gain equalizer 515 cannot be effectivelyperformed.

[0175] To cope with this problem, such a change in the gain infront-stage amplifier section 501 is transmitted to middle-stage ALCsection 503 and rear-stage amplifier section 505 to change the ALCreference value in middle-stage ALC section 503 and the set gain inrear-stage amplifier section 505, thereby compensating for thegain-wavelength characteristic of the optical amplifier as a whole. Thatis, the voltage value Vagc expressed as a logarithmic value of the gainGf' in front-stage amplifier section 501 is output from subtracter 523,and the voltage value Vagc is then subtracted from the AGC gainreference value Vagcref by subtracter 524. The difference obtained bysubtracter 524 is then supplied as the gain correction value Vadj toboth middle-stage ALC section 503 and rear-stage amplifier section 505.

[0176] In middle-stage ALC section 503, the ALC reference value Valcrefis changed according to the gain correction value Vadj from front-stageamplifier section 501. For example, in the case that the gain infront-stage amplifier section 501 decreases by 1 dB, the gain correctionvalue Vadj=1 is input to subtracter 537, and the difference obtained bysubtracting the gain correction value Vadj from the ALC reference valueValcref, i.e., the difference of (Valcref−1) dB is supplied as a new ALCreference value Valcref' from subtracter 537 to ALC circuit 534. Then,the optical attenuation of variable optical attenuator 530 is controlledaccording to this new ALC reference value Valcref', so that the WDMsignal light having a level lower by 1 dB than the level maintainedunder the constant gain control in front-stage amplifier section 501 isoutput from middle-stage ALC section 503 to rear-stage amplifier section505.

[0177] In rear-stage amplifier section 505, the set gain in AGC ischanged according to the gain correction value Vadj from front-stageamplifier section 501. In the above example, the gain correction valueVadj=1 from front-stage amplifier section 501 is input to adder 561, andadded to the voltage value expressed as a logarithmic value suppliedfrom log transformer 560. That is, the voltage value obtained byincreasing the level of the WDM signal light to be supplied to EDF 554by 1 is supplied from adder 561 to anti-log transformer 562. Then, thevoltage value obtained as an anti-logarithmic value from anti-logtransformer 562 is supplied to AGC circuit 559, thereby amplifying theWDM signal light with a gain Gr′ higher by 1 dB than the gain Gr. Whenthe gain is increased, the gain-wavelength characteristic of EDF 554changes so as to have a negative tilt as shown in FIG. 20, so that thecompensation of EDF 554 by gain equalizer 551 cannot be effectivelyperformed. However, the uncompensated amount of the gain-wavelengthcharacteristic in front-stage amplifier section 501 is canceled inrear-stage amplifier section 505, thereby allowing effectivecompensation for the gain-wavelength characteristic of the opticalamplifier as a whole.

[0178] At this time, an increase in pump light power required toincrease the gain in rear-stage amplifier section 505 can be suppressedto a relatively small amount by operating EDF 554 in a saturated region.This is due to the fact that the relation of gain to input light levelin an EDF generally has a negative tilt of about −0.8 in a saturatedregion as shown in FIG. 23 (e.g., when the input light level decreasesby 1 dB, the gain increases by about 0.8 dB). Accordingly, an increasein pump light power required to increase the set gain can be suppressed.

[0179] As described above, in the case that the input light level ishigh, the gain Gf in front-stage amplifier section 501 is changed to thelower gain Gf' without restriction by AGC, thereby eliminating the needfor use of a large-capacity light source as pumping source 518, and itis sufficient for pumping source 518 to supply pump light in itsattainable range of output power to EDF 513. Even when thegain-wavelength characteristic of EDF 513 cannot be effectivelycompensated by gain equalizer 515 because of a change in gain infront-stage amplifier section 501, the gain-wavelength characteristicsin front-stage and rear-stage amplifier sections 501 and 505 can becounterbalanced with each other by changing the ALC reference value inmiddle-stage ALC section 503 and the set gain in rear-stage amplifiersection 505, thereby obtaining WDM signal light whose channels areequalized in level. Further, by operating rear-stage amplifier section505 in a saturated region, an increase in pump light power in rear-stageamplifier section 505 can be minimized in spite of a decrease in gain infront-stage amplifier section 501. Regarding noise characteristics,there is little need for especially reducing a noise figure to ensure anS/N ratio at a receiving end, because the input light level is high.Accordingly, it can be said that it is almost unnecessary to maintain ahigh gain in front-stage amplifier section 501 and reduce a noise figurein comparison with the case that the input light level is low.

[0180] In FIG. 21, the gain-wavelength characteristic in front-stageamplifier section 501 and the gain-wavelength characteristic inrear-stage amplifier section 505 change with the same tendency inresponse to a change in the set gains. That is, the Er doped fiberamplifiers used in front-stage amplifier section 501 and rear-stageamplifier section 505 are similar in configuration to each other.However, the present invention is not limited to this configuration. Forexample, even in the case that the amounts of change in thegain-wavelength characteristics in the front-stage and rear-stageamplifier sections are different from each other in response to a changein the set gains, it is sufficient that the tendencies of change in thegain-wavelength characteristics in the front-stage and rear-stageamplifier sections are to be the same as each other, thereby exhibitingthe effect of counterbalancing the gain-wavelength characteristics inthe front-stage and rear-stage amplifier sections according to the abovepreferred embodiment. In this case, however, precise compensation forthe gain-wavelength characteristic in the optical amplifier as a wholeis not assured. Further, also in the case that the tendencies of changein the gain-wavelength characteristics in the front-stage and rear-stageamplifier sections are opposite to each other in response to a change inthe set gains, a similar effect can be obtained by using a subtracter inplace of the adder 561 in rear-stage amplifier section 505, for example.

[0181] While erbium doped fibers 513 and 554 are used in front-stage andrear-stage amplifier sections 501 and 505, the present invention is notlimited to this configuration. For example, rare earth doped fiberscontaining any rare earth elements other than erbium may also be used infront-stage and rear-stage amplifier sections 501 and 505. Further,while the optical amplifier in FIG. 21 has a two-stage amplifyingconfiguration, three or more-stage amplifying configuration may beadopted.

[0182]FIG. 24 is a diagram illustrating an optical communication systemaccording to embodiments of the present invention. Referring now to FIG.24, optical transmitters (TX1 . . . TXN) 600 transmit optical signals atdifferent wavelengths (λ₁. . . λ_(N)), respectively. A multiplexer (MUX)610 multiplexes the optical signals into a WDM signal light, andprovides the WDM signal light to an optical fiber transmission line 620.An optical amplification device 630 amplifies the WDM signal light asthe WDM signal light is travels through the transmission line 620. Ademultiplexer (DEMUX) 640 demultiplexes the WDM signal light so that theindividual optical signals can be received by receivers (RX1 . . . RXN)650. Optical amplification device 630 can have a configuration asdescribed herein for any of the embodiments of the present invention.For example, optical amplification device 630 can have a configurationas disclosed, for example, in either FIGS. 5, 7, 8, 9, 10, 12, 13, 15 or21.

[0183] According to the above embodiments of the present invention, anoptical amplification device (such as an optical amplifier) for WDMincludes a gain deviation detecting device and a reference gain changingdevice. Accordingly, even in the case that the level of input light tothe optical amplification device largely changes, the wavelengthdependence of gain of each optical amplification stage can be reliablycompensated without restriction by constant gain control. As a result,the optical amplification device can output WDM signal light whosechannels are equalized in level. Further, there is no need for using alarge-capacity pumping source to maintain the constant gain control,thereby attaining a cost reduction. Further, noise characteristics canbe improved because the gain of the front-stage optical amplifyingdevice can be set to a higher value. In addition, because a constantlevel control device and a reference level changing device are provided,it is possible to obtain WDM signal light whose optical signals in allthe channels are amplified with a uniform gain and a constant level.Thus, it is possible to provide an optical amplifier for WDM having morestable amplification characteristics.

[0184] According to the above embodiments of the present invention, anoptical amplification device (such as an optical amplifier) forwavelength division multiplexing includes a front-stage amplifiersection 501 and a rear-stage amplifier section 505 each for amplifyingWDM signal light having changing input level under constant gaincontrol, and a middle-stage ALC section 503 for controlling the WDMsignal light to a constant level. In front-stage amplifier section 501,a gain correction value Vadj indicative of a gain deviation is detectedby log transformers 521 and 522 and subtracters 523 and 524. Accordingto the gain correction value Vadj, an ALC reference value Valcref inmiddle-stage ALC section 502 and a reference gain in rear-stageamplifier section 505 are changed. Accordingly, a change in thegain-wavelength characteristic of front-stage amplifier section. 501 iscanceled by the gain-wavelength characteristic of rear-stage amplifiersection 505, thereby effectively compensating for the gain-wavelengthcharacteristic of the optical amplification device as a whole.

[0185] According to the above embodiments of the present invention,there is provided in an optical amplification device for WDM. Aplurality of optical amplifying devices are cascaded together, eachhaving a rare earth doped fiber for amplifying WDM signal light byreceiving pump light. A plurality of constant gain control devicescontrol the power of the pump light so that the gain of each opticalamplifying device becomes a predetermined reference gain. Again-wavelength characteristic compensating device compensates for thewavelength dependence of gain of each optical amplifying device at thereference gain. Moreover, a gain deviation detecting device detects again deviation between the gain of each optical amplifying device andthe reference gain. A reference gain changing device changes thereference gain so that when the gain deviation of at least one of theplurality of optical amplifying devices is detected by the gaindeviation detecting device, the reference gain of the other opticalamplifying devices whose gain deviation is not detected is changed tocancel a change in the wavelength dependence of gain generated in theoptical amplifying device whose gain deviation is detected.

[0186] With this configuration, the WDM signal light input to theoptical amplification device is sequentially amplified by the pluralcascaded optical amplifying devices. The pump light power in eachoptical amplifying device is controlled by the constant gain controldevice to thereby maintain the gain of each optical amplifying deviceconstant in an attainable output range of the pump light. When theconstant gain control for each optical amplifying device is maintained,the wavelength dependence of gain in each optical amplifying device iscompensated by the gain-wavelength characteristic compensating device tothereby obtain WDM signal light whose channels are equalized in level.When the control by the constant gain control device in each opticalamplifying device becomes out of the attainable output range of the pumplight, the constant gain control for the optical amplifying devicecannot be maintained to result in generation of a deviation between thegain of the optical amplifying device and its reference gain. As aresult, the wavelength dependence of gain in the optical amplifyingdevice changes to cause a problem that the compensation by thegain-wavelength characteristic compensating device does not effectivelyfunction. To cope with this problem, when this gain deviation isdetected by the gain deviation detecting device, the reference gain ofthe other optical amplifying devices whose gain deviation is notdetected is changed by the reference gain changing device according tothe detected gain deviation. As a result, the change in the wavelengthdependence of gain of the optical amplifying device whose constant gaincontrol cannot be maintained is canceled by the wavelength dependence ofgain of the other optical amplifying device whose reference gain hasbeen changed, thereby obtaining WDM signal light whose channels areequalized in level.

[0187] Accordingly, even in the case that the level of input light toeach optical amplifying device largely changes, the wavelengthdependence of gain of each optical amplifying device can be reliablycompensated without restriction by the constant gain control, therebyallowing output of WDM signal light having a flat gain-wavelengthcharacteristic as the whole of the optical amplifier. Accordingly, theneed for especially using a large-capacity pumping source can beeliminated to thereby reduce the costs of the optical amplifier for WDM.Further, the gain of the front-stage amplifying device can be set to ahigher value, so that noise characteristics can also be improved.

[0188] Preferably, an optical amplification device according toembodiments of the present invention further comprises a level adjustingdevice, a constant level control device and a reference level changingdevice. The level adjusting device is provided on the front stage orrear stage of the plurality of optical amplifying devices or between theplurality of optical amplifying devices, for adjusting the level of theWDM signal light. The constant level control device controls anadjusting amount by the level adjusting device so that the power levelof the WDM signal light per channel becomes a predetermined constantreference level. The reference level changing device changes thereference level so that when the gain deviation is detected by the gaindeviation detecting device and the level adjusting device is provided onthe rear stage of the optical amplifying device whose gain deviation isdetected, the reference level is changed according to the gain deviationdetected.

[0189] With this configuration, WDM signal light whose optical signalsof all the channels are amplified with the same gain to a constant levelcan be obtained irrespective of whether or not the constant gain controlis maintained for each optical amplifying device. Accordingly, it ispossible to provide an optical amplifier for WDM which can be morestabilized in amplification characteristics.

[0190] Preferably, the gain deviation detecting device comprises a gainmeasuring device for measuring the gain of any one of the plurality ofoptical amplifying devices in which gain control by the constant gaincontrol device cannot be maintained in a range of change in input lightlevel, and gain deviation computing device for comparing the gainmeasured by the gain measuring device and the reference gain to obtainthe gain deviation. The reference gain changing device is provided forany one of the plurality of optical amplifying devices in which the gaincontrol by the constant gain control device can be maintained in therange of change in input light level.

[0191] According to the above embodiments of the present invention, thegain measuring device measures the gain according to the level of theWDM signal light input to the optical amplifying device and the level ofthe WDM signal light output from the optical amplifying device. As amodification, the gain measuring device may measure the gain accordingto the level of amplified spontaneous emission generated in the rareearth doped fiber of the optical amplifying device.

[0192] According to the above embodiments of the present invention, whenthe wavelength dependence of gain of the plurality of optical amplifyingdevices change with the same tendency in response to a change in gain,the reference gain of the other optical amplifying devices is increasedwith a decrease in gain in the optical amplifying device whose gaindeviation is detected, whereas the reference gain of the other opticalamplifying device is decreased with an increase in gain in the opticalamplifying device whose gain deviation is detected.

[0193] Preferably, the gain deviation detecting device outputs alogarithmic value of the gain deviation, and the reference gain changingdevice changes the reference gain by using the logarithmic value of thegain deviation and the reference gain. More preferably, the referencelevel changing device changes the reference level by using thelogarithmic value of the gain deviation and the reference level.

[0194] Therefore, according to the above embodiments of the presentinvention, an optical amplifier for use in a WDM optical communicationsystem can ensure a required input dynamic range, can compensate for thegain-wavelength characteristic of each optical amplifying device withinthe optical amplifier without restriction by constant gain control, andcan improve noise characteristics.

[0195] Various of the above embodiments of the present invention relateto an optical amplification device, such as an optical amplifier or anoptical repeater, which has two optical amplifiers, or two opticalamplification stages, cascaded together. However, the embodiments of thepresent invention are not intended to be limited to opticalamplification devices having two stages. Instead, the present inventionis applicable to apparatuses and methods in which more than two opticalamplifiers, or optical amplification stages, are cascaded together. Asan example, according to embodiments of the present invention asdescribed above, an apparatus can include a plurality of opticalamplifiers cascaded together so that a light is amplified by eachoptical amplifier as the light travels through the cascaded plurality ofoptical amplifiers. Each optical amplifier amplifies the light with acorresponding gain. A gain adjustor detects a deviation in gain of oneof the plurality of optical amplifiers from a target gain, and adjuststhe gain of at least one of the other of the plurality of opticalamplifiers to compensate for the detected deviation. Here, the pluralityof optical amplifiers cascaded together can include two or more opticalamplifiers cascaded together.

[0196] Various wavelengths, frequencies and/or other numerical examplesare provided herein to describe optical signals, pump lights, wavelengthbands, etc. The present invention is not intended to be limited to thesewavelengths, frequencies and/or other numerical examples.

[0197] Although a few preferred embodiments of the present inventionhave been shown and described, it would be appreciated by those skilledin the art that changes may be made in these embodiments withoutdeparting from the principles and spirit of the invention, the scope ofwhich is defined in the claims and their equivalents.

What is claimed is:
 1. An apparatus comprising: a first opticalamplifier receiving a light and amplifying the received light; a secondoptical amplifier receiving the light amplified by the first opticalamplifier, and amplifying the received light; and a controller which,when a level of the light received by the first optical amplifierchanges by Δ, controls a level of the light received by the secondoptical amplifier to change by approximately −Δ.
 2. An apparatus as inclaim 1, further comprising: a feedback loop maintaining constant alevel of light output from at least one of the group consisting of thefirst and second optical amplifiers.
 3. An apparatus as in claim 1,further comprising: a first feedback loop maintaining a level of thelight amplified by the first optical amplifier to be constant; and asecond feedback loop maintaining a level of the light amplified by thesecond optical amplifier to be constant.
 4. An apparatus as in claim 1,wherein the controller comprises a variable attenuator opticallyconnected between the first and second optical amplifiers, thecontroller controlling attenuation of the variable attenuator to controlthe level of the light received by the second optical amplifier.
 5. Anapparatus as in claim 2, wherein the controller comprises a variableattenuator optically connected between the first and second opticalamplifiers, the controller controlling attenuation of the variableattenuator to control the level of the light received by the secondoptical amplifier.
 6. An apparatus as in claim 3, wherein the controllercomprises a variable attenuator optically connected between the firstand second optical amplifiers, the controller controlling attenuation ofthe variable attenuator to control the level of the light received bythe second optical amplifier.
 7. An apparatus as in claim 3, whereineach of the first and second optical amplifiers includes an opticalamplifying medium, and a light source supplying pump light to theoptical amplifying medium so that the optical amplifying medium providesa gain band including a wavelength included in light amplified by therespective optical amplifier, the first feedback loop maintains a levelof the light amplified by the first optical amplifier to be constant bycontrolling a power of the pump light supplied by the light source ofthe first optical amplifier, and the second feedback loop maintains alevel of the light amplified by the second optical amplifier to beconstant by controlling a power of the pump light supplied by the lightsource of the second optical amplifier.
 8. An apparatus as in claim 1,wherein each of the first and second optical amplifiers includes anoptical fiber doped with a rare earth element; and a light sourcesupplying pump light to the optical fiber so that the optical fiberprovides a gain band including a wavelength included in light amplifiedby the respective optical amplifier.
 9. An apparatus as in claim 1,wherein, to change by approximately −Δ, the level of the light receivedby the second optical amplifier changes by −(Δ±Δ/20).
 10. An apparatuscomprising: a first optical amplifier receiving a light and amplifyingthe received light; a second optical amplifier receiving the lightamplified by the first optical amplifier, and amplifying the receivedlight; a variable attenuator optically connected between the first andsecond optical amplifiers; and a controller controlling attenuation ofthe variable attenuator so that, when a level of the light received bythe first optical amplifier changes by Δ, a level of the light receivedby the second optical amplifier changes by approximately −Δ.
 11. Anapparatus as in claim 10, further comprising: a feedback loopmaintaining constant a level of light output from at least one of thegroup consisting of the first and second optical amplifiers.
 12. Anapparatus as in claim 10, further comprising: a first feedback loopmaintaining a level of the light amplified by the first opticalamplifier to be constant; and a second feedback loop maintaining a levelof the light amplified by the second optical amplifier to be constant.13. An apparatus as in claim 10, wherein, to change by approximately −Δ,the level of the light received by the second optical amplifier changesby −(Δ±Δ/20).
 14. An optical communication system comprising: opticaltransmitters which transmit optical signals at different wavelengths; amultiplexer multiplexing the optical signals into a wavelength divisionmultiplexed (WDM) signal light; an optical fiber transmission linethrough which the WDM signal light is transmitted; and an opticalamplification device amplifying the WDM signal light as the WDM signallight is transmitted through the transmission line, the opticalamplification device including a first optical amplifier receiving theWDM signal light and amplifying the received WDM signal light, a secondoptical amplifier receiving the WDM signal light amplified by the firstoptical amplifier, and amplifying the received WDM signal light, and acontroller which, when a level of the WDM signal light received by thefirst optical amplifier changes by Δ, controls a level of the WDM signallight received by the second optical amplifier to change byapproximately −Δ.
 15. An optical communication system as in claim 14,wherein the optical amplification device further comprises: a feedbackloop maintaining constant a level of WDM signal light output from atleast one of the group consisting of the first and second opticalamplifiers.
 16. An optical communication system as in claim 14, whereinthe optical amplification device further comprises: a first feedbackloop maintaining a level of the WDM signal light amplified by the firstoptical amplifier to be constant; and a second feedback loop maintaininga level of the WDM signal light amplified by the second opticalamplifier to be constant.
 17. An optical communication system as inclaim 14, wherein the controller comprises a variable attenuatoroptically connected between the first and second optical amplifiers, thecontroller controlling attenuation of the variable. attenuator tocontrol the level of the WDM signal light received by the second opticalamplifier.
 18. An optical communication system as in claim 14, furthercomprising a plurality of said optical amplification devices positionedalong the transmission line to amplify the WDM signal light as the WDMsignal light is transmitted through the transmission line.
 19. Anapparatus as in claim 14, wherein, to change by approximately −Δ, thelevel of the WDM signal light received by the second optical amplifierchanges by −(Δ±Δ/20).
 20. A method comprising: a first amplificationprocess of receiving a light and optically amplifying the receivedlight; a second amplification process of receiving the light amplifiedby the first amplification process, and optically amplifying thereceived light; and, when a level of the light received by the firstamplification process changes by Δ, controlling a level of the lightreceived by the second amplification process to change by approximately−Δ.
 21. A method as in claim 20, further comprising: maintainingconstant a level of light amplified by at least one of the groupconsisting of the first and second amplification processes.
 22. A methodas in claim 20, further comprising: maintaining a level of the lightamplified by the first amplification process to be constant; andmaintaining a level of the light amplified by the second amplificationprocess to be constant.
 23. A method as in claim 20, wherein saidcontrolling comprises controlling attenuation of the light received bythe second amplification process, to thereby control the level of thelight received by the second attenuation process.
 24. A method as inclaim 20, wherein, to change by approximately −Δ, the level of the lightreceived by the second amplification process changes by −(Δ±Δ/20). 25.An apparatus comprising: optical amplifiers optically connectedtogether, each optical amplifier having a corresponding gain; and acontroller causing the sum of the gains of the optical amplifiers to beconstant.
 26. An apparatus as in claim 25, further comprising: avariable attenuator optical connected between respective opticalamplifiers of said optical amplifiers, wherein the controller controlsattenuation of the variable attenuation so that the sum of the gains ofsaid optical amplifiers is constant.
 27. An apparatus comprising: afirst optical amplifier receiving a light and amplifying the receivedlight with a gain of the first optical amplifier; a second opticalamplifier receiving the light amplified by the first optical amplifier,and amplifying the received light with a gain of the second opticalamplifier; and a controller causing the sum of the gains of the firstand second optical amplifiers to be constant.
 28. An apparatus as inclaim 27, further comprising: a feedback loop maintaining constant alevel of light output from at least one of the group consisting of thefirst and second optical amplifiers.
 29. An apparatus as in claim 27,further comprising: a first feedback loop maintaining a level of thelight amplified by the first optical amplifier to be constant; and asecond feedback loop maintaining a level of the light amplified by thesecond optical amplifier to be constant.
 30. An apparatus as in claim27, wherein the controller comprises a variable attenuator opticallyconnected between the first and second optical amplifiers, thecontroller controlling attenuation of the variable attenuator to causethe sum of the gains of the first and second optical amplifiers to beconstant.
 31. An apparatus as in claim 29, wherein the controllercomprises a variable attenuator optically connected between the firstand second optical amplifiers, the controller controlling attenuation ofthe variable attenuator to cause the sum of the gains of the first andsecond optical amplifiers to be constant.
 32. An apparatus as in claim27, wherein the controller comprises: a first gain monitor detecting thegain of the first optical amplifier in accordance with an input leveland an output level of the first optical amplifier in a given wavelengthband; and a second gain monitor detecting the gain of the second opticalamplifier in accordance with an input level and an output level of thesecond optical amplifier in a given wavelength band, the controllercausing the sum of the gains of the first and second optical amplifiersto be constant in accordance with the gains detected by the first andsecond gain monitors.
 33. An apparatus as in claim 27, wherein each ofthe first and second optical amplifiers includes an optical fiber dopedwith a rare earth element; and a light source supplying pump light tothe optical fiber so that the optical fiber provides a gain bandincluding a wavelength included in light amplified by the respectiveoptical amplifier.
 34. An apparatus as in claim 33, wherein thecontroller comprises: a first gain monitor detecting the gain of thefirst optical amplifier in accordance with an absorption of pump lightin the doped optical fiber of the first optical amplifier; and a secondgain monitor detecting the gain of the second optical amplifier inaccordance with an absorption of pump light in the doped optical fiberof the second optical amplifier, the controller causing the sum of thegains of the first and second optical amplifiers to be constant inaccordance with the gains detected by the first and second gainmonitors.
 35. An apparatus as in claim 33, wherein the controllercomprises: a first gain monitor detecting the gain of the first opticalamplifier in accordance with a power of spontaneous emission lightemitted from the doped optical fiber of the first optical amplifier; anda second gain monitor detecting the gain of the second optical amplifierin accordance with a power of spontaneous emission light emitted fromthe doped optical fiber of the second optical amplifier, the controllercausing the sum of the gains of the first and second optical amplifiersto be constant in accordance with the gains detected by the first andsecond gain monitors.
 36. An apparatus comprising: a first opticalamplifier receiving a light and amplifying the received light with again of the first optical amplifier; a second optical amplifierreceiving the light amplified by the first optical amplifier, andamplifying the received light with a gain of the second opticalamplifier; a variable attenuater optically connected between the firstand second optical amplifiers; and a controller controlling attenuationof the variable attenuator to cause the sum of the gains of the firstand second optical amplifiers to be constant.
 37. An apparatus as inclaim 36, further comprising: a feedback loop maintaining constant alevel of light output from at least one of the group consisting of thefirst and second optical amplifiers.
 38. An apparatus as in claim 36,further comprising: a first feedback loop maintaining a level of thelight amplified by the first optical amplifier to be constant; and asecond feedback loop maintaining a level of the light amplified by thesecond optical amplifier to be constant.
 39. An optical communicationsystem comprising: optical transmitters which transmit optical signalsat different wavelengths; a multiplexer multiplexing the optical signalsinto a wavelength division multiplexed (WDM) signal light; an opticalfiber transmission line through which the WDM signal light istransmitted; and an optical amplification device amplifying the WDMsignal light as the WDM signal light is transmitted through thetransmission line, the optical amplification device including opticalamplifiers optically connected together, each optical amplifier having acorresponding gain, and a controller causing the sum of the gains of theoptical amplifiers to be constant.
 40. An apparatus as in claim 39,wherein the controller comprises: a variable attenuator opticalconnected between respective optical amplifiers of said opticalamplifiers, wherein the controller controls attenuation of the variableattenuation so that the sum of the gains of said optical amplifiers isconstant.
 41. A method comprising: providing optical amplifiersoptically connected together, each optical amplifier having acorresponding gain; and causing the sum of the gains of the opticalamplifiers to be constant.
 42. A method comprising: a firstamplification process of receiving a light and optically amplifying thereceived light with a gain of the first amplification process; a secondamplification process of receiving the light amplified by the firstoptical amplifier, and optically amplifying the received light with again of the second amplification process; and causing the sum of thegains of the first and second amplification processes to be constant.43. A method as in claim 42, further comprising: maintaining constant alevel of light amplified by at least one of the group consisting of thefirst and second amplification processes.
 44. A method as in claim 42,further comprising: maintaining a level of the light amplified by thefirst amplification process to be constant; and maintaining a level ofthe light amplified by the second amplification process to be constant.45. An apparatus comprising: a first optical amplifier amplifying alight with a gain of the first optical amplifier; a second opticalamplifier receiving the light amplified by the first optical amplifier,and amplifying the received light with a gain of the second opticalamplifier; and a gain adjustor detecting a deviation in gain of thefirst optical amplifier from a target gain, and adjusting the gain ofthe second optical amplifier to compensate for the detected deviation.46. An apparatus as in claim 45, further comprising: a first gaincontroller controlling the gain of the first optical amplifier to beconstant; and a second gain controller controlling the gain of thesecond optical amplifier to be constant.
 47. An apparatus as in claim45, further comprising: a first compensating device compensating forwavelength dependence of gain of the first optical amplifier; and asecond compensating device compensating for wavelength dependence ofgain of the second optical amplifier.
 48. An apparatus as in claim 46,further comprising: a first compensating device compensating forwavelength dependence of gain of the first optical amplifier; and asecond compensating device compensating for wavelength dependence ofgain of the second optical amplifier.
 49. An apparatus as in claim 45,further comprising: a level controller controlling a level of the lightamplified by the first optical amplifier before being amplified by thesecond optical amplifier to be at a target level.
 50. An apparatus as inclaim 45, further comprising: a level controller controlling a level ofthe light amplified by the first optical amplifier before beingamplified by the second optical amplifier to be at a target level, thelevel controller adjusting the target level to compensate for thedetected deviation in gain of the first optical amplifier.
 51. Anapparatus as in claim 46, further comprising: a level controllercontrolling a level of the light amplified by the first opticalamplifier before being amplified by the second optical amplifier to beat a target level, the level controller adjusting the target level tocompensate for the detected deviation in gain of the first opticalamplifier.
 52. An apparatus as in claim 45, wherein the gain adjustorcomprises: a gain detecting device detecting the deviation in gain ofthe first optical amplifier; and a gain adjusting device adjusting thegain of the second optical amplifier in accordance with the detecteddeviation.
 53. An apparatus as in claim 45, further comprising: a gaincontroller controlling the gain of the first optical amplifier to beconstant, wherein the deviation in gain of the first optical amplifieris a deviation from the target gain caused by a change in power level ofthe light before being amplified by the first optical amplifier.
 54. Anapparatus as in claim 50, further comprising: a gain controllercontrolling the gain of the first optical amplifier to be constant,wherein the deviation in gain of the first optical amplifier is adeviation from the target gain caused by a change in power level of thelight before being amplified by the first optical amplifier.
 55. Anapparatus as in claim 52, wherein the gain detecting device detects thegain of the first optical amplifier from input and output levels of thefirst optical amplifier, and detects the deviation in gain from thedetected gain.
 56. An apparatus as in claim 52, wherein the firstoptical amplifier comprises a doped optical fiber which amplifies thelight as the light travels through the doped optical fiber, and the gaindetecting device detects the gain of the first optical amplifier fromamplified spontaneous emission generated in the doped optical fiber, anddetects the deviation in gain from the detected gain.
 57. An apparatusas in claim 45, wherein, to compensate for the detected deviation ingain of the first optical amplifier, the gain adjustor increases thegain of the second optical amplifier to compensate for a decrease ingain of the first optical amplifier, and decreases the gain of thesecond optical amplifier to compensate for an increase in gain of thefirst optical amplifier.
 58. An apparatus as in claim 45, wherein thegain adjustor comprises: logarithmic circuits detecting a logarithmicvalue corresponding to the deviation in gain of the first opticalamplifier, the logarithmic value being used to adjust the gain of thesecond optical amplifier.
 59. An apparatus comprising: a first opticalamplifier amplifying a light with a gain of the first optical amplifier;a first gain controller controlling the gain of the first opticalamplifier to be constant at a target gain; a second optical amplifierreceiving the light amplified by the first optical amplifier, andamplifying the received light with a gain of the second opticalamplifier; a gain deviation detector detecting a deviation in gain ofthe first optical amplifier from the target gain; and a gain adjustoradjusting the gain of the second optical amplifier to compensate for thedetected deviation in gain of the first optical amplifier.
 60. Anapparatus as in claim 59, further comprising: a level controllercontrolling a level of the light amplified by the first opticalamplifier before being amplified by the second optical amplifier to beat a target level, the level controller adjusting the target level tocompensate for a detected deviation in gain of the first opticalamplifier by the gain deviation detector.
 61. An apparatus as in claim59, further comprising: a second gain controller controlling the gain ofthe second optical amplifier to be constant.
 62. An apparatus as inclaim 59, further comprising: a first compensating device compensatingfor wavelength dependence of gain of the first optical amplifier; and asecond compensating device compensating for wavelength dependence ofgain of the second optical amplifier.
 63. An apparatus as in claim 60,further comprising: a first compensating device compensating forwavelength dependence of gain of the first optical amplifier; and asecond compensating device compensating for wavelength dependence ofgain of the second optical amplifier.
 64. An optical amplifying devicecomprising: a first optical amplifier amplifying a light with a gain ofthe first optical amplifier; a first gain controller controlling thegain of the first optical amplifier to be constant at a target gain forthe first optical amplifier; a second optical amplifier receiving thelight amplified by the first optical amplifier, and amplifying thereceived light with a gain of the second optical amplifier; a secondgain controller controlling the gain of the second optical amplifier tobe constant at a target gain for the second optical amplifier; a gaindeviation detector detecting a deviation in gain of the first opticalamplifier from the target gain of the first optical amplifier; and again adjustor adjusting the target gain of the second optical amplifierto compensate for the detected deviation in gain of the first opticalamplifier.
 65. An optical amplifying device as in claim 64, furthercomprising: a level controller controlling a level of the lightamplified by the first optical amplifier before being amplified by thesecond optical amplifier to be at a target level, the level controlleradjusting the target level to compensate for the detected deviation ingain of the first optical amplifier.
 66. An optical amplifying device asin claim 65, further comprising: a first compensating devicecompensating for wavelength dependence of gain of the first opticalamplifier; and a second compensating device compensating for wavelengthdependence of gain of the second optical amplifier.
 67. An apparatuscomprising: a first optical amplifier amplifying a light with a gain ofthe first optical amplifier; a second optical amplifier receiving thelight amplified by the first optical amplifier, and amplifying thereceived light with a gain of the second optical amplifier; and a gainadjustor detecting a deviation in gain of one of the first and secondoptical amplifiers from a target gain, and adjusting the gain of theother of the first and second optical amplifiers to compensate for thedetected deviation.
 68. An apparatus as in claim 67, further comprising:a first gain controller controlling the gain of the first opticalamplifier to be constant; and a second gain controller controlling thegain of the second optical amplifier to be constant.
 69. An apparatus asin claim 67, further comprising: a first compensating devicecompensating for wavelength dependence of gain of the first opticalamplifier; and a second compensating device compensating for wavelengthdependence of gain of the second optical amplifier.
 70. An apparatus asin claim 68, further comprising: a first compensating devicecompensating for wavelength dependence of gain of the first opticalamplifier; and a second compensating device compensating for wavelengthdependence of gain of the second optical amplifier.
 71. An apparatus asin claim 67, further comprising: a level controller controlling a levelof the light amplified by the first optical amplifier before beingamplified by the second optical amplifier to be at a target level, thelevel controller adjusting the target level to compensate for thedetected deviation in gain.
 72. An apparatus as in claim 68, furthercomprising: a level controller controlling a level of the lightamplified by the first optical amplifier before being amplified by thesecond optical amplifier to be at a target level, the level controlleradjusting the target level to compensate for the detected deviation ingain.
 73. An apparatus comprising: a first optical amplifier amplifyinga light with a gain of the first optical amplifier; a first gaincontroller controlling the gain of the first optical amplifier to beconstant at a target gain for the first optical amplifier; a secondoptical amplifier receiving the light amplified by the first opticalamplifier, and amplifying the received light with a gain of the secondoptical amplifier; a second gain controller controlling the gain of thesecond optical amplifier to be constant at a target gain for the secondoptical amplifier; and a gain adjustor detecting a deviation in gain ofone of the first and second optical amplifiers from its target gain, andadjusting the gain of the other of the first and second opticalamplifiers to compensate for the detected deviation.
 74. An apparatus asin claim 73, further comprising: a first compensating devicecompensating for wavelength dependence of gain of the first opticalamplifier; and a second compensating device compensating for wavelengthdependence of gain of the second optical amplifier.
 75. An apparatus asin claim 73, further comprising: a level controller controlling a levelof the light amplified by the first optical amplifier before beingamplified by the second optical amplifier to be at a target level, thelevel controller adjusting the target level to compensate for thedetected deviation in gain.
 76. An apparatus comprising: a plurality ofoptical amplifiers cascaded together so that a light is amplified byeach optical amplifier as the light travels through the cascadedplurality of optical amplifiers, each optical amplifier amplifying thelight with a corresponding gain; and a gain adjustor detecting adeviation in gain of one of the plurality of optical amplifiers from atarget gain, and adjusting the gain of at least one of the other of theplurality of optical amplifiers to compensate for the detecteddeviation.
 77. An apparatus as in claim 76, further comprising: aplurality of gain controllers corresponding, respectively, to theplurality of optical amplifiers, each gain controller controlling thegain of the corresponding optical amplifier to be constant.
 78. Anapparatus as in claim 76, further comprising: a level controllerpositioned between the respective optical amplifier for which thedeviation in gain is detected and a respective optical amplifier ofwhich the gain is adjusted, the level controller controlling a level ofthe light to be at a target level, the level controller adjusting thetarget level to compensate for the detected deviation in gain.
 79. Amethod comprising: a first optical amplification process of amplifying alight with a gain of the first optical amplification process;controlling the gain of the first optical amplification process to beconstant at a target gain; a second optical amplification process ofreceiving the light amplified by the first optical amplificationprocess, and amplifying the received light with a gain of the secondoptical amplifying process; detecting a deviation in gain of the firstoptical amplification process from the target gain; and adjusting thegain of the second optical amplification process to compensate for thedetected deviation.
 80. An optical communication system comprising:optical transmitters which transmit optical signals at differentwavelengths; a multiplexer multiplexing the optical signals into awavelength division multiplexed (WDM) signal light; an optical fibertransmission line through which the WDM signal light is transmitted; andan optical amplification device amplifying the WDM signal light as theWDM signal light is transmitted through the transmission line, theoptical amplification device including a first optical amplifieramplifying the WDM signal light with a gain of the first opticalamplifier, a second optical amplifier receiving the WDM signal lightamplified by the first optical amplifier, and amplifying the receivedWDM signal light with a gain of the second optical amplifier, and a gainadjustor detecting a deviation in gain of the first optical amplifierfrom a target gain, and adjusting the gain of the second opticalamplifier to compensate for the detected deviation.